I pull a trailor also. I assume that due to a weak charging system that the flashers draw too much voltage to allow the flashing mechanism to engage. I think that it takes quite a bit of voltage to operate. I'm just guessing. If you find out otherwise please post it.
ps I'm thinking that the extra draw on the system by the running lights on the trailor creates this situation. I need to make that clear. 4-12-07: I just updated all my trailor lights to led's As I suspected the reduced elec. load and walah the 4 way flashers work like a charm and the turn signals don't sound like they have a load on them anymore. Don't need the isolator others have proposed any more. My main purpose for changing to led's was to free up the weak charging system on the 2nd gen vr and a 70% red. in electricity can't hurt. The led's are brighter and maintenance free. I was constantly having to wiggle or adjust bulbs to keep them lit.
