Before you try anything else check your brushes first, they are wear items
and are real easy to fix.  I that doesn't work try the following.  Now I'm
assuming your 85 is like my 88,  if so then the sense line is one of the
three wires coming from the plug in the side of the alternator.  The
heaviest wire is the one that goes back to the battery and supplies charging
current to the electrical system.  Of the two smaller ones one goes to the
warning light, and the other is the voltage sensing line.  Try disconnecting
one of the smaller wires and see what happens.  If your warning light comes
on you got the wrong one.  Once you find the right one, a down and dirty fix
is to hook up the sense line to the positive side of the battery.  Don't
hook the warning light wire up by mistake or you will let all the magic
smoke out of the box.

Since you don't have an internally regulated alternator, ignore my previous
post.  Like the man said, remove the regulator and have it tested.  You can
also perform a down and dirty test yourself by measuring the voltage from
the positive terminal of the battery to ground.  Have someone rev the engine
for you while you look at the voltmeter, output should remain constant at
around 14 volts.  If the voltage changes radically or is just plain too high
then your regulator is suspect.  If the regulator checks good then we are
back to the possibly of a wiring problem.