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I did a wiring / relay harness upgrade on my '86 740; "rolled my own" (so to speak) rather than buy ready-made, though. Given the time & effort both in rounding up the parts and assembly, I'd say in general that $70 isn't a bad price, though. If you're not comfortable with making the harness from scratch or don't have the time available, then buying ready-made is the way to go.
Overall, I would say the decision would be based on two things:
First - are you using (on intend to use) e-code lights with or without high-watt bulbs? If the answer is "Yes", install the harness/relay assembly.
Second - if you are using stock bulbs & lights, what is your voltage drop at the bulbs? IMHO, if it's more than 1/2 volt, clean up the connections (don't forget the ground); if it's still low, install the harness/relay assembly.
I did it because I'm using 100w/80w H4 and 130w H1 bulbs (I have the quad rectangular lights)- the OEM wiring would self-destruct under those loads. Nevertheless, I think it's worthwhile even with standard bulbs; higher voltage to the bulbs & less current going through the old OEM wiring & switches, thus probably prolonging their life.
By doing my own, I was able to use the components I preferred (wire gauge, insulation color, relays, fuse block, etc.) and soldered all the terminals, rather than rely solely upon crimped connections. I also used a food storage box - similar to Tupperware(TM) - to house the relays & fuse block, fastening it inside the engine compartment near the battery. The fuse block was fed with 8 gauge wire from the battery (+) terminal, with 14 gauge wire from relays to each filament, as well from the bulb to ground (each bulb has its own ground wire).
Relays are Bosch 40 amp made for VW/Audi; standard pinout & size, common and inexpensive (used them because I had a bunch on hand). To keep the "bulb-out" sensor happy, the low beam relay coil is fed from both OEM low-beam circuits, with diodes in-line to isolate each one from the other - that way the load is the same on both sides, and the "bulb-out" light stays off. Found the diodes at Radio Shack, used 2A ones with the lowest voltage drop I could find; the drop across the diodes is about 3/4 volt, IIRC - the relay coil still gets adequate voltage to operate without problems. (My testing showed the relays draw about 650 mA, and would drop out at 5.5-6 volts. YMMV, depending on which relays you use.)
Also, I was able to wire the H1 bulbs as separate driving lights, even using a Volvo OEM driving light switch (catalog calls it a "Spot Light" switch).
Biggest problem I had was finding push-on connectors to match the size on the bulbs; all aftermarket headlight sockets I could find were crimped with what looked like 18 gauge wire, and seemed too light-duty for my uses. Finally found them at a local electrical / electronics supply house. Only downside to using push-on terminals is that one must pay attention to what wire goes where when replacing the H4 bulbs.
It's not rocket science, just basic electrics.
--
Tom Harper ('86 740GLE; NØMKC; Denver, Colorado)
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