DC motor caps
There are a couple of reasons for the caps. For one, they tend to reduce some of the commutator arcing and wear on the motor. In tape decks especially, they're there to reduce the amount of eletrical noise that would otherwise get into the audio amplifiers.
Get in your car, tune the car radio to a dead spot on the AM band and then play with the heater blower speed switch. You'll likely hear a whine that varies with the blower speed. This is what the caps are trying to get rid of.
Dean