We thought it was just us. Our '01 TJ, Red, has been through three or four blower-motor switches in its life and two resistor blocks (the part that controls the speed of the blower motor). We just figured that because Red was off-road a lot and used in places where the blower motor was constantly going, we were bound to burn some out. It wasn't until Trasborg's girlfriend, with her '06 LJ, told us about possible dashboard fires from just using the heat that we went looking into the problem.
As these things often turn out, a few days after she mentioned the fire issue, we started smelling that acrid electrical fire smell. In the course of our one-hour commute, we had lost all the speeds on the blower motor. This, of course, prompted a dashboard teardown in the office parking lot and revealed a fried blower-motor switch. The thing was severely cooked, and it came with a side of burnt wires on the mode-selector switch that Cappa conveniently forgot to tell Trasborg about when he handed the keys over.
After looking into it, we've found that many '99-and-later TJ owners are experiencing loss of control when it comes to the HVAC system in the Jeep, and looking at our bubbled, melted plastic parts just flat-out scared us. Out of the factory, the heater worked fine. The issue is, as the Jeep is driven-on-road and off-road-the blower motor wears down. As the brushes in the motor wear, the motor itself is subjected to more dirt and debris, and eventually, it will require more (in our case, many more) amps just to turn than it did when it left the factory.
In fact, it takes so many amps, the blower-motor switch and associated wiring just isn't rated to handle it. Like all electric things when they're overloaded, the switch heats up-when that happens, more resistance is created, more juice is required, and so on.
So we got to thinking. With factory switches running at $30 a pop, why not add a relay or two and reduce the amperage being drawn through the switch itself? We would let the relays (at $5 or $6 each) pass the high current the blower motor may or may not need and save the switch. We went down to the local electronics emporium and grabbed a handful of relays and prewired relay plugs to make the whole thing easier and wired them into our HVAC system to prevent more heater-control meltdowns.
Basically, the electrical side consists of three major components: the blower-motor switch, the blower-motor resistor, and the blower motor itself. The mode switch is also involved in switching ground, but on its own doesn't get nearly as hot as the blower switch.
What we did was cut the four wires that go from the switch to the resistor block and put a relay in the middle of the wire. The switch and resistor block work together to vary the resistance to ground and reduce the speed of the blower. The blower motor has two wires going into it: one that goes to a switched positive and another that goes through the resistor block, through the blower-motor switch, and then to ground.
Follow along as we wire up our relays to switch the ground circuit so the blower-motor switch doesn't have to anymore.
The fun ball of melted plastic on the left is switch No. 3 to have been installed in this Jeep. The unit on the right is brand-new from Chrysler, and it's what the switch should look like out of the box. | 
Here's the joy that Cappa left us. This plug was supposed to have three wires in it. The truth was the wires were barely holding the plug into the range-selector switch, and if we hit a bump, it would no longer make contact. | 
We ended up with not just one relay for each of the two higher speeds, but we went with the total overkill of one relay for each speed and are still only out about $40. |
The back of the mode-selector switch looked OK once we got the melted wire mess off it and scraped off some of the excess plastic. We've heard that to replace any of the connectors that are attached to the controller unit can be very expensive, and because the mode selector looked OK, we extended the wires just enough to work with, reconnected them using spade connectors, covered it with liquid electrical tape, and wrapped it with regular tape. We figured with the clean, solid connections, there should be no more melting issues here. | 
Somewhere in the middle of all this, we decided to figure out exactly how hot the connection was getting. This is the connector for the blower-motor switch, and we jumped it into the high position with a 12-gauge wire. After running it on the high setting for an hour, with an ambient temperature of 70 degrees F, we shot it with our infrared thermometer and saw a shocking 122 degrees at the connector. After the swap, the connector was at 80 degrees in the 78-degree ambient temperature of the Jeep, while the relays were at a reasonable 116 degrees. | 
Wire to blower motor switch (one side of cut wire)Switched, fused battery positiveWire to resistor block (other side of cut wire)To groundThis is the wiring with terminal numbers for the relays we put in. Repeat this four times, and you'll have meltdown-resistant HVAC controls. |