You need to know the parts numbers, which are stamped on your amp, of the units that will work.
There is probably more than one model number that will work, but if you can get the same number it will work for sure.
The only practical way to see if the amp is good is to stick it in the car. The actuator and a speedo relay can also cause problems, I understand.
It will cost more than $50 to fix this, as it requires rather expensive equipment and a considerable amount of a technician's time to get it to work,
Thomaspin on this forum has a description of a fix on his excellent website that might help.
Semibodacious Transmogrifications a Specialty
1990 300D 2.5 Turbo sedan 171K (Rudolf)
1985 300D Turbo TD Wagon 219K (Remuda)
"Time flies like and arrow, yet fruit flies like a banana"