The cassette player has 2 belts which drive everything. A common problem with cassette belts are that they dissolve into a black goop when they get old. The goop gets everywhere inside the mechanism and fowls it up.
It takes some careful cleaning with acetone to remove the goop.
The blue arrows point to the belts
The yellow arrow points to anther drive wheel with its rubber tire
The green arrow points the the players flywheel, which keeps everything moving along nicely