The Renix blower motor has a long rambling ground wire on it; all the way to the other side of the engine compartment. A relay on the blower motor hot wire isn't going to help much if at all with a poor ground. Cut off your existing black ground wire near the motor connector, and using a butt splice (or solder & heat shrink), splice it back together including an additional length of 10 - 12 AWG wire. Tie this new wire to a good grounding point on the right fender, or even on the dipstick tube ground cluster. Your blower motor will run faster. It would also be a good idea to pull your blower motor first and check the ducting for any obstructions inside.
For the 91s and 92s, the blower motor ground configuration was upgraded by the factory and is satisfactory as-is.