DIY Ford Mustang EFI wiring harness conversion- GETTING Oct 28, 2012DIY Ford Mustang EFI wiring harness conversion- GETTING UGLY. Has anyone seen a step-by-step guide for operating on the factory MAF EEC-IV harness? Would there be any benefit in doing one, if one doesn't exist. I think I'm going to take the plunge on mtfit's old 77 and am planning to use a factory uncut engine harness. Right now I'm just DIY Ford Mustang EFI wiring harness conversion- GETTING Oct 28, 2012use harness tape and loam to wrap up the harness NOT Electrical tape, RJM sells it 3 rolls will do it get 5 just in case. RJM harness is a good deal, all new parts buy it and drop it in. I would not spend any money on paying someone else to rework an old harness figure $300 rework + $100 avg for a used harness you might as well buy an RJM harness.