Had exactly the same problem with my W124 260e a couple of months ago. Cost to do complete overhaul was $ 1,600.00 Aus dollars. Bought a low k box and changed over for $ 900.00 Aus dollars.

The W124 260e and w201 190e 2.6 share the same transmission. The W124 300 M103 engine has a different transmission.

The overhaul specialist gave me this opinion. Repairing the reverse gearing components will only delay the forward gear mechanism repair. His opinion was that it probably is good money thrown away just repairing the reverse mechanism.

It probably depends on the age of the car and how many K's you have done whether or not this is a cost effective job.

My transmission pan has this gunk in it. Coloured red from the trans fluid.