Relays are used to switch higher currents. It's easier to switch 15A with a small switch energizing a coil with a beefy set of contacts than to design a switch that has a light touch and 15A contacts.

A starter solenoid is an example where a very small contact in the key tumbler switches very high current, on the order of 100A-400A. Without the solenoid, the switch would have contacts so big you probably could not turn the key using both hands!

You need at least two relays for each motor.
-One (forward) is wired +/+, -/- (battery positive to motor positive) to spin the motor one direction.
-One (reverse) is wired +/- and -/+ to run it backwards.

You wire battery positive to the common of the switch. Pressing in one direction closes the 'forward' contact and energizes the 'forward' relay. Pressing in the other direction closes the 'reverse' contact, and energizs the 'reverse' relay.

I don't know how the W124 seat is wired, but this is a general overview of the theory.

I hope this helps.