Sorry for the delay in posting to your question.
As far as adapting an aftermarket brain and plugging it into the wiring harness, you could probably do just that and get it to work. However, I must caution you on a couple things.
First, the 240 cars were not only NOTORIOUS, but also completely INFAMOUS for melting their fuse boxes and other wiring. As it was, the wiring is RIVETED to the fuse box, and I've tried soldering some of these and only ended up with melted plastic around the connection. Adding wire and other current drawing devices seems to me like putting 3 overweight big guys in a boat rated for 4 "normal" sized guys, yes TECHNICALLY you haven't overloaded it cause only 3 guys are in there, but you have exceeded the weight limitation, go ahead and fish, I'll wait on shore.
Second, are you good and I mean REALLY good at reading schematics? If not, then why put yourself through the headache of trying to figure out where to splice into the old harness, when the engineers figured it out for you on a later harness.
Yes, removing the dash, and other parts that hide where the wiring harness was snaked through may seem like a lot of trouble, but believe me, it isn't all that hard.
I've personally replaced the complete wiring harness on the back of my dash. In so doing I eliminated several "improvements" by the prior owner that resulted in several short circuits, a melted fuse box and also a shorted ammeter (try that one out for size) as well as a burned out accessory relay. Now, the lights on the dash light, they dim, the heater fan works, the pre-heater on the heater box works, the tach works again, the turn signals show the proper side, the brake warning light operates properly, the horn works, the CLOCK works, and most important of all, the battery actually maintains its charge. Was it a PITA, yes, would rather have been driving it. Would I do it again, you bet! Now I can access either the Chilton's, Hayne's, FSM, Clymer's or other wiring diagram and know exactly where and what wires to work with to add in the fog lamps to the actual circuit included in the wiring harness.
So let's face it, unless you are incredibly methodically anal about annotating each and EVERY connection for your aftermarket brains, if you have any problems, you will be starting from close to ground zero every time you have a problem. Additionally, going to a mechanic? Only if you've been there before and he knows that it used to work and doesn't question your splicing into it. It is much easier to say, it's a 71 with a 78 wiring harness and turbo, so use the 78 wiring schematic and tune up specs.
Sorry for the ranting, but I think you'll learn more about your car by doing the harness swap than trying to splice in something. In fact, it might even save you a headache or two.
FWIW
Enrique Scanlon
eM:
[email protected]