I'm not speaking with broad MB experience.

With you in the seat the motor draws much more current. There could be a marginal electrical connection to the motor. At low current flow (unloaded seat) there is little voltage drop across this marginal connection. With the seat loaded current flow increases and the voltage drop across the connection is great enough that there isn't enough voltage across the motor.

Find what connections you can. Before disconnecting them inspect them to see if they are visually seated properly. Then diconnect them and inspect the contacts for arcing. Also, make sure the contacts are clean.

Maybe you'll get lucky.