Turn signal/hazzard/headlight issue

[92 Dime]

Got an electrical problem. '86 GMC 2wd. Found out today I had no rear lights: no turn sgnals, no brakelights. These went out about the time I turned the headlights on in a rain storm.

Checked fuses. None blown.

Checked bulbs. Two were burnt out, so replaced them all. No change.

Removed, cleaned, reinstalled ground connection on the rear frame. That got me a dim glow on the filaments. At one time there was a trailer connector splice in, but this was later cut to disconnect. I removed the electrical tape wrapping it all: water poured out. Got that drying now.

Tried the hazzard lights to see if that had any effect on the turn signals. They were subjectively a little dimmer. I think this was a mistake, as now when I try the turn signals, they function as hazzard lights. Left turn, right turn, doesn't matter: every light that can flash, does.

With the headlights on, I have no lights in the back.

Swapping out the flashers with known-good flashers had no effect on this.

Any recommendations on trouble-shooting from here? Pretty sure (but not certain) that rat's nest where the trailer connector was spliced in is involved. Should I start there?

Where are the grounds from the cab back? (Maybe a better way to put that would be "where should the grounds be from the cab back?")

Worst case scenario, if I splice in new wires from the cab back, any recommendations on wire gauge and number/location of new grounds?

thanks guys. I hate electrical problems.