I think I have figured out how to determine if a 420SEL power seat has any defective motors or a defective seat control module.

There are three motors under the seat, which control only the seat bottom's pitch, height and fore/aft. Each of these three motors transmits its power to the seat-adjusting mechanism through its own square cable in a housing (3 cables in housings).

There are two smaller motors in the seat back that control the head restraint and the seat back recline. (If you need to access them, you can reach them without removing the seat, by removing two screws on the bottom of the rear of the seat back and removing the rear of the seat back. But you can operate them without removing that - see below.)

The connectors to each motor have two large pins, 1 and 2; the rest of the pins are smaller. Pins 1 and 2 connect to the motor windings and receive 12 volts when they are supposed to operate. You can test each motor by disconnecting it from the seat control module and applying 12 volts to pins 1 and 2 of its connector, through a 25-amp fuse. To reverse the motion of the motor you are testing, reverse the polarity of the 12 volt supply. You don't really need to remove the seat to do this, if you are a contortionist as most of us old M-B freaks are.

If all the motors work, then the problem is in the seat control module or the switch (assuming you have ruled out fuses and wiring harness). Remove the seat, turn it upside down on a bench, and then remove the seat control module and examine its underside, looking for the telltale bulges/swellings/cracks in the plastic housing covering the circuit board that indicate a circuit board trace drew too much current, overheated and burned up.

If the seat control module looks OK, you are going to have to measure the supply voltages at the small pins on the connectors to determine if the seat control module is getting the correct voltage supply from the seat switch on the door panel. You will have to follow the schematic in the service CD to do this.

If you're getting the correct supply voltages from the door panel switch, then you've got to swap in a different seat control module and see if that makes it work.

But if you can't get one of the motors to work when you apply power to it directly, and you need to adjust the seat in order to drive the car, remove all the motors and lay them aside until you can repair them. Now you will have the 3 square cables exposed and extending out of their housings. Chuck the end of the cable into an electric drill and GINGERLY make the adjustment by triggering the drill. Be careful you don't wind the cable up against the mechanism's end of travel. If you are concerned about using an electric drill, use a hand-powered "egg beater" drill instead.

My problem seems to be in the seat control module. I can see a burned spot through the bottom of the plastic housing. Question: These modules aren't sold on an exchange basis, are they? So is there any reason why I might not want to try disassembling the seat control module and repairing the circuit board trace?

I guess I am pretty dumb, but after posting all of the above verbiage I discovered that the real problem is that the ratcheting or pawl mechanism that has to engage in order for the seat back to move upright isn't engaging. The recline motor runs in both directions. I can't figure out how to disassemble the seat back from the seat bottom in order to gain access to the recline pawl or ratchet mechanism.

Any suggestions? Do I have to drill out rivets at this point?