Switching the leads of a high current motor
To switch the polarity you need either one DPDT relay to switch direction PLUS a SPST relay to turn the juice on or off, or a pair of SPDT relays. For the latter, if both relays are pulled in or dropped out nothing happens. Pulling one or the other in drives the motor in one direction or the other. 







simulate this circuit &ndash; Schematic created using CircuitLab

Semiconductor switching of traction motors is not for the faint of heart- you would likely need to massively overrate the switching devices to keep them from failing. That's certainly my experience. And failing 'on' with a wheelchair motor could conceivably be dangerous- propelling the hapless operator unexpectedly into some place they don't want to be.