1st gen turn signal troubles.

So I swapped out the bed on my hunting truck recently. Of course, this involved removing all wires that were connected to the lights.
I put everything back like it was afaik.

Now I have a strange issue going on. (Driver side, everything works correctly.)
Passenger side- blinker operate normally, until I press the brake, then the blinker flashes rapidly. Let off the brake, and it goes back to normal. Also, if the tail lights are on, I get that rapid blink, only on the passenger side.

Has anyone seen this before? I've tried several things without any luck. I added a ground wire from the replacement bed to the frame as well.

Even stranger happenings- I park this truck inside nearly all the time. When it's parked outside, and I drive it first thing in the morning, the problem goes away for the first 15-20 mins of driving. (I'm thinking this is because of the morning dew.) It sometimes works properly while driving in the rain as well.

Thanks in advance y'all!

Fast fwd to this afternoon--


I just worked on them for a couple of hours. They're not LED bulbs, just standard, and I re-used the ones from my original bed.

I took everything apart, used a meter to try and figured out which wires were grounds. I blew 3 or 4- 10a fuses but I finally got the turn signals working properly, with/without the brake and lights on/off.

Now that's fixed, here's another huge pita, haha.

I only have a working tail light on the driver side, and only the passenger side brake light works. That's messed up, I wouldn't believe it, actually if I didn't see it myself.

I need to move on with my day, will give it another shot after I get some other things taken care of.