Weird electrical problem., light hook, surge brakes

Thanks for your time.  95 Suburban 350 4wd.  It is wired for a trailer with either 4 or 5 prong connector.  We just got a new boat trailer, and it uses 5 prongs (my previous trailer used 4 prongs but the new one uses 5 to deactivate the surge brakes - I'm not certain the trailer light hook-up is part of the problem).  The truck and trailer lights worked fine together. But about a day after I got home and disconnected the trailer my wife noticed the brake lights weren't working (except for the 'third' brake light at the top!), and neither were the turn signals and 4-ways.  I checked the fuses, relays and replaced the flasher cube with a heavy duty type, but no change.  A shop said it is the 'multi function switch' in the steering column and wants $634.  Does his diagnosis sound right?  Thanks again Mike.

Answer
    It is almost certainly the trailer plug causing the problem.  I would get a new harness that plugs directly into the truck's wiring harness.  Inspect the old wiring as you remove it to make sure that there are no shorts or broken wires.  It is usually a ground problem, but not necessarily.  The fact that the high mount stop light still works is your clue that the harness is bad, the harness plugs in downstream of the connection to the HMSL.  After you fix everything, try and find a friend with a trailer that you know works, and plug your rig into his trailer to make sure that everything's OK.  If it works on his trailer and doesn't on yours, then you'll know there is a problem with your trailer.  Again, almost always a bad ground, caused by corrosion.
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