www.w116.org has the FSM available for free.
here:
http://handbook.w116.org/matrix6.htm
If you have the 722.120 transmission that you are supposed to have ( or any 722.1xx four bolt pan), you HAVE a reverse band adjustment. Not sure this will fix your problem, but don't let anyone touch your transmission until he/she finds it, adjusts it and confirms that your band is kaput. The link I provided earlier has a good shot of what to look for. It's on the passenger's side, just behind the bellhousing. It is above (and hidden by ) a protruding servo-type thing ( I think it may be the kickdown)that intersects the case above the oil pan at a diagonal.
If you are not concerned about A/C in summer, you can find inventive ways to bypass the servo with a simple valve to provide heat. Yes, the amplifier looks to be easy to replace, but buried behind the glovebox. The servo probably isn't too hard either, just a lot of connections to keep straight.
Rick