elctrical problem need help asap

ok heres my problem, i got a 87 b2200. and when ur use the turn siginals they work fine, but as soon as u turn the lighs on and use the turn siginals the turn lights flash dim and the tail lights will flash as well. ive checked all the bulbs and they seem to all be fine. there has been some electrical work done to it, but all wires are going in right spots. anyone have any ideas?

try changing the relays

grounds! check em all

mike is right^^^ check the grounds its most likely at the plug on the tail light

and again! GROUNDS. Gaurantee it!

thanks guys, i ended up checking the grounds on the rear, and it ended up being the two grounds that screw to the frame, so i just cleaned em up and screwed em in a new spot and works perfect! thanks for the help.