This would be a good mod to do:
CRUISER'S MOSTLY RENIX TIPS
IMPROVING BLOWER MOTOR PERFORMANCE
NOVEMBER 28, 2015 CRUISER54 28 COMMENTS
On 1984 to 1990 MJs and XJs, the blower motor’s factory grounding point is on the driver side inner fender under the sheet metal screw. This ground is shared with windshield wipers, front windshield washers, rear windshield washers, AC clutch relay, fan control relay, fog lamps, fan motor, headlamps, front turn signals, front side markers, and park lamps.
So your blower motor has its ground point 10 feet away from where it is located!!
What we’re going to do is leave that ground intact and also ground the blower motor on the passenger side inner fender much closer to the blower motor itself. This will also benefit the other components on the factory ground circuit. Take this opportunity to refresh the factory ground as a matter of course. Remove the screw, scrape the surface to bare metal and reinstall the screw securely.
Here’s what I do to get the ground much closer to the blower motor and add another ground point to this overloaded ground circuit.
Find the blower motor connector on the passenger side. Red and Black two wire connector.
Find a location where the black wire can be made to reach the passenger side inner fender, and cut the wire. You may have to do some rerouting of the harness to achieve this.
Take both cut pieces of wire and put them together into a yellow eyelet and crimp. Fasten the eyelet to a place on the passenger side inner fender with a sheet metal screw after applying OxGard to the contact surfaces. Be sure to scrape the attaching point on the fender to bare metal first.
Your blower motor will now turn faster and last longer, and the other electrical components on the circuit will benefit from a better ground path.