I have successfully installed a used MSD 6A ignition box on my '86 XJ with a 2.5L engine. I have read that a few people seem to poo-poo the idea but with the improved cold idle when first starting it, I am quite pleased it.
The MSD install wasn't straight forward, but it isn't bad once you know what needs to be done. This post is here so other might benefit from what I found out. Pay attention to the pictures if my text isn't clear of what I did.
The first picture shows where I mounted the box; right side of engine compartment, just above the coil. I had some sheet steel that I folded in 2 spots - attached one side in 2 spots to the inner fender and the other side down to a tab that held 2 relays. I painted it silver to match the fading silver of my XJ.
The second picture is for reference as to how it look in relation to the coil. Note the white ballast resistor.
The third picture is the interesting part. The coil on my XJ screws to an aluminum mount. This mounting bracket has 2 sockets for 2 plugs from the wiring harness. When the coil when removed it shows 2 more connections/plugs that plug into 2 more sockets on the aluminum mount/bracket. The bracket/mount has some electronics potted on the underside of it so you need to keep this piece. This makes things more difficult, but not impossible.
In order for the MSD box to work, you need to intercept the signal from the ECM, route it into the MSD and back out to the coil. The red/white wires from the MSD connect to the sockets on the coil mount (think distributor/ECM). Polarity is important. The orange/black wires from the MSD connect to the coil. Again polarity is important.
I made 2 supports to mount the coil higher in the mount using 2 pieces of 1/4 inch copper tubing. They were about 3/4 of an inch. It cuts easy and can be sanded flat on the ends. Steel tubing could have worked, but I didn't have any. I also used the same copper tubing to make 2 pieces that would plug into the sockets in the coil mount.
The socket pieces were just a hair over 1/4 inch long. I put a small machine screw through the tubing piece and slightly flattened the tubing onto it. The coil mount needs 2 holes drilled in order for the bolt to get out. I pushed the flattened copper tube pieces into the sockets and the threads out the holes. The red and white wires will connect to these bolts. I think they were 8x32 and about an inch long. (Might be 6/32?) I used what was on hand. The coil has 2 round brass 'plugs' that snaps into the mount. On my coil, those brass pieces were threaded on to 2 screws that came out of the coil. I simply unscrewed the brass pieces and attached the orange and black wires there. POLARITY IS IMPORTANT!
Sharp eyed readers will see an extra red wire connected at the coil mount. This red wire connects to a Chrysler Dual Ballast Resistor. Picture 2 shows this ballast resistor. 4 spade lugs; the first spade connects with the red wire of the MSD box, the second spade connects to the second resistor (third spade) and the other side of this ballast resistor (fourth spade) connects to ground. When both sides connected together it measured 72 ohms with my cheapo Wally World DVM. This is important as my XJ continued to run after the key was turned off. The ballast resistor drains the excess voltage off to ground when the key is turned off, but has enough resistance not to effect the operation of the MSD box. It shuts off like is supposed to now.
The violet and green wires from the MSD box had the ends cleared and a piece of heat shrink applied to keep them tidy.

My total cost for this is under $50. I found the MSD 6A ignition box on Craigslist for $40. The ballast resistor I picked up at O'Reillys for $7.00. The nuts, bolts, heat shrink, connectors, etc., I had on hand. The time I spent researching all of this wasn't measured, but I spent more than one evening looking at pictures, PDFs, websites before I started this project. The best part is, it works! And now that I figured this all out, and can work for you too.

 

Disculpa a dónde conectaste el cable de los rpm