MRM Modified V8 Miata Conversion Harness

Cheer for your wiring woes are over! Introducing the MRM Modified Conversion Harness!

· Includes a 21-page installation manual with over 40  photos!


· Includes a completely custom Fuel Pump Relay circuit.


· Completely eliminates the need to “fool” the computer into not triggering the check engine light.

Price: $600

(+core charge-see below)

Feeling a bit daunted by the prospect of modifying your own Mustang harness for your Ford Miata conversion? Having trouble finding a good harness to modify? Don’t go and spend $600 or $700 for a new one, send your Mustang harness to us and we’ll modify it for you. After we receive your harness we start by stripping off all the old tape and split loom and completely cleaning each individual wire. Once that’s done we inspect the harness and ether repair of replace any damaged portions. Then we remove any unused wires and plugs and reroute the remaining ones so that the harness will literally “drop-in” to a Mazda Miata. We then add a circuit to allow the Ford ECU to run the Mazda fuel pump without the need of a jumper circuit and create a connection for the factory VSS (speed sensor). Lastly we label each and every connection so you know exactly where to hook it into your Miata’s existing wires.


New for 2016:

We are now happy to announce that we can also create a MRM conversion harness for you using an SN95 donor harness. The cost is the same ($600) as our Fox-body conversion.


Core Charge Explanation:

If you don’t have a Mass-Air Mustang harness to send us we’ll find you a donor harness in a local salvage yard or on eBay!  Typically good, unmolested Mass-air Mustang harnesses go for about $150 to $200 on eBay. Another option is to go and purchase a harness yourself and have it shipped directly to us.


Download the Install Manual HERE!!



See! Wiring your Monster doesn’t need to be scary...

To Order email or call the following:




Phone: 267.254.6687





Who is MRM?




Video Links

Random Pics

McCully Racing Motors

Because two heads are ALWAYS better than one.

Other Harness-Related Services:

In addition to modifying your main harness, we can also clean up your other needed harnesses for you, or even integrate them in if you do not have them!

HEGO (O2 Sensor) Harness clean-up: $35.00


Includes complete strip-down and inspection, removal of oil-level wires (not needed in a Miata swap), replacement of any damaged plugs with NOS, full cleaning, rewrapping and labeling.

HEGO (O2 Sensor) Integration: $35.00


Didn’t get a HEGO Harness with your donor? Don’t feel like paying $60 or more on eBay for an old beat up one? No problem! We can integrate the HEGO connections into your main harness so that rather than a HEGO harness plug your converted main harness will include two new O2 sensor plugs! We label them right and left and route leads down the shifter hole branch with the speed sensor lead so that the HEGO wires never enter the hot engine bay.

TFI Branch Reroute: FREE (with purchase of a TFI Kit with your Conversion Harness)


As normally configured, the TFI branch of a Mustang’s main harness runs from the firewall, under the upper intake, and to the TFI Module on the distributer. If a remote TFI kit is added at a later date you are limited to placing it within reach of that branch. If you order a TFI kit at the same time as your conversion harness not only do we drop the $10.01 S/H on the kit but also we’ll reroute and lengthen the TFI lead of your main harness so that it can run as far as in front of the radiator!




Injection Harness clean-up: $35.00


Includes complete strip-down and inspection, removal of oil pressure and HEGO ground wires (both routed through main harness in a Miata swap), replacement of any damaged plugs with NOS, full cleaning, replacement of H2O temp sender plug with Mazda-compatible plug, rewrapping and labeling.




All relays are relocated to one location behind the firewall (out of the engine bay). A fuel pump relay and its circuit is added to the harness.

The HEGO (o2 sensor) harness and the Fuel injection harness are integrated into the main harness, eliminating the troublesome “salt and pepper” shaker connections.

Smog wires are removed from the harness and pulled behind the firewall and sealed. They are labeled should one decide to use one as a output on a chipped ecu or need to wire a cheat device.

Under hood and under dash connections are clearly labeled for Miata or stand alone use.