I’m repainting my 2003 Ranger with 2 bulb taillights and replaced the bed with a 1996 that has 3 bulb taillights. The 1996 bed harness plugs into the 2003 harness and I get taillights but no brake lights or turn signals. Any ideas?

 

You need to put your 2003 taillights and wiring harness back in.

 

+1 ^^^

Ranger had 2 tail light wiring designs, 2000 and up were "2 wire", older were "3 wire"
On older the 3rd wire was for separate turn signal bulb
In 2000 Ford combined turn/brake on same wire/bulb, for 2 wire

So either change the lens's to 2 wire or buy a 2 to 3wire converter, used to wire trailer lights, not 3 to 2, but 2 to 3, there is both