Went to AutoZone and bought the exact replacement plugs, and a new serpentine belt. Replacing the plugs took a long time because I first had to learn how to unplug the ignition connector, then I had to learn how to remove the coil bar from the engine; and then I had to remove the old plugs.
One at a time I pulled the new plugs out of their boxes, checked the gap, put anti-seize compound on the threads, and put 'em in. Then I had to install the coil bar twice because once I got it in place the first time I discovered that I couldn't plug it into the wiring harness. Argh etc.
But once I got everything together, I started the truck and ran it long enough to ensure there were no misses--there weren't--and then went to work on replacing the serpentine belt.
The tensioner in the Cherokee is kind of different from what I'm used to. Instead of a spring-loaded idler, the idler pulley has an adjustment bolt. But I couldn't get the thing to move, so I had to finagle and fiddle before I finally figured out that I had to loosen the bolt in the center of the pulley as well in order to get it to move. Then it was pretty simple after that.
But in the process of replacing the serpentine belt I unplugged the coolant temperature sensor, so I had a better shot at the tensioner bolt, and I forgot to plug it back in before starting the truck. With it running I saw that the sensor was unplugged and plugged it in; but when I was all done, the "check engine" light was on. Argh.
...so I took it to AutoZone to get the codes read, hoping it was just the coolant sensor and not something else. Fortunately it was the coolant sensor.
I also bought a stereo mounting kit at AutoZone, $10, but I need to buy the correct wiring adaptor as well. That shouldn't be too bad. I tried diagnosing the problem with the stereo but didn't get very far. I swapped the rear speakers and found that no matter whcih speaker was plugged into the left side, it still sounded muddy. Hopefully that means it's in the head unit.
The front driver's side speaker emits no sound at all. The heat unit is--or should be, at any rate--the only thing the two speakers have in common.
But the problem is, the Haynes manual says that in the "typical" radio installation there is an auxiliary amplifier, but I'm switched if I can figure out where the hell it is, or if it's even installed at all. I have the owner's manual and the build sheet for the truck, and that's it; I have no idea what options the thing has unless they are bleeding obvious, like tilt, cruise, power windows, etc.
Apparently you could get an upgraded sound system by Infinity; that might be what the manual is referring to.
Anyway I can get the factory service manual (FSM) for $90 plus S&H from Chrysler; maybe I ought to.