[JimD]

I assume the "bulbs out" occurs when the trailer lights are connected. If not, I do not know how to "fix it". If my guess is correct and it's only when the lights are connected, you might try changing the jet ski trailer to led lights. I have them on my boat and like them better for a couple reasons. First, they do not corrode and loose the contact when the trailer sit unused. Second, they draw very little power. Third they do not get hot so nothing bad happens if you do not disconnect them before dunking them in the lake.

Even though your setup is not supposed to take it's power from the light circuit, my guess is that it is to some degree and dropping back a lot on the current may eliminate the bulb out signal. Even if it does not, the led lights have advantages.

I just used a cheap set from harbor freight. They were $40 or less. They work fine.

Jim