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How to replace BMW e82 hood release cables.
Symptom:
Can't open hood from inside vehicle. Hood latch becomes increasingly difficult to release, eventually the cable becomes slack and the hood release will no longer function. The issue is the cable has become stretched and will no longer activate the hood release. BMW can charge you upwards of $850 to do this. Save yourself the cash, it takes about an hour to do this job.

Fix:
Replace both cables that work in unison to release the hood latch. Replace both cables at the same time, they are cheap parts and it better to do both at once.
The part numbers are: 51-23-7-201-904 and 51-23-7-184-603.
51-23-7-201-904 is the cable that runs through the firewall and attaches to hood release lever inside the vehicle.
51-23-7-184-603 is the cable that connects to the hood release latch, it has the black plastic connector box where you attach the 51-23-7-201-904 cable that runs into the vehicle. The black plastic box flips open and inside there is a simple connection between the front and back cables.
NOTE: Before removing anything note the routing and attachment points of the old cables so you know where to place the new cables.

Removal:
1) Remove both front kidney grilles, this will allow access to the front cable and the release latch that it is attached to. You can grab the cable with pliers and release the hood manually to open it once the kidney grilles are out. Open the hood and disconnect the rear cable from the front cable by opening the black box and disconnecting the cables. The box hinges, you just have to pry it open with your fingers or a small screwdriver.
2) Trace the rear cable note that it runs into the firewall, you will need to remove the items that obstruct your work. These items are easily removed, the brake distribution block cover, the screen that catches leaves and the rubber funnel that the screen sits in. All of these items can be removed by hand. The rubber funnel has two plastic pawls that need to be removed, one on the fender wall, one fender wheel well. Once these items are removed you can see where the cable enters the firewall.
3) Inside the vehicle, remove the driver’s door sill trim, there are about six or eight plastic pawls to pop off. Remove the drivers footwell plastic kick plate. Use a Phillips screwdriver to remove the hood release lever and the screw that is underneath it. There are two plastic pawls on this piece, pop them out and disconnect the trunk release wire harness that connects to the trunk release button. Set the kick plate aside and remove the end of the cable from the small plastic wheel that has a slot where the wire rests and metal end tab is inserted. Pull the cable cover out of the seat where it sits, go under the hood and pull the cable out of the firewall, set it aside.
4) The front cable is attached to the hood release latch; you can’t detach the cable until you remove the hood release latch. There are two Torx bolts that attach the release latch to the body of the vehicle. Loosen these two bolts, lower the release latch and remove the cable from the latch. Run the new cable exactly where the old cable was routed, there are three slotted cable holders that the front cable fits into, the cable pushes into these holders, it is pretty simple setup when you get in there.

Installation
1) Take the rear cable and smear some dish soap on the rubber gasket that will seal the firewall penetration. Smearing liquid dish soap will allow the rubber gasket to slip into the firewall hole more easily. Push the cable through the firewall hole until you have pushed the rubber gasket through the opening, don’t push too far, you want to make sure the collar on the gasket is seated correctly. I pushed the gasket into the hole and slowly pulled it out until the gasket collar was seated correctly. It is pretty easy to get it to seal, just pull slowly and keep looking at your work, eventually it will seat correctly. While you are doing this check to see if you can see the cable inside the vehicle.
2) Go inside the car, you should see the cable sticking out, if it is not obvious, look under the carpet. The cable only has a few places to go and it does not penetrate very far into the vehicle. Attach the tab that is at the end of the cable into the black wheel where you removed the old cable. Insert the cable housing into the seat where the end of old cable housing was attached. It pushes into the seat with a small amount of force.
3) Go under the hood and route the rear cable in the same place the old rear cable was routed. Make sure it is routed to the black box that is attached to the front cable.
4) Route the front cable in the same place as the old front cable, don’t attach it to the body yet. Attach the end of the front cable to the hood release latch that you removed earlier. There is a black plastic collar that has a mount point, push the black plastic collar into the mount attach the metal end to the hood latch. Reattach the hood release latch with the Torx bolts, push the cable into the body attachment mounts that are behind the grill openings. Push the black box into the holes in the body where the plastic pins insert. Attach the rear cable to the front cable inside the black box, close the black box. Check your work to make sure the hood can be released from inside the car. Reinstall everything in reverse order and you are done!!