Your not "upgrading" the car. Your making an inferior wiring harness that will look bad and may cause unexpected problems with the car. If you can't afford to fix your car correctly and balk at the price of parts why not look at a cheaper car? Would you stick a VW bug engine into it if you threw a rod? Maybee a GM 2 speed transmission if you needed a transmission rebuild?

Im far from a purist but if I opened the hood and saw some cobbled together wiring harness under the hood the first thing I would do is shut the hood and leave. Your absolutely killing the resale value of your car that way. Many of the people who specifically look for 140's know 140's so your market would be first time MB buyers that did not know what to look for.

The new wiring harness is a different kind of wiring that does not break down over time. If you want a real scare just wait 5-7 years when the wiring harness on the inside of your car starts to break down