[Occasionally6]

The cleanest way is to make a patch harness i.e. a short extension that plugs in between the two - male and female - connectors that are already on the car. That may be easier said than done.

If you want to do it at the ECU, the harness end is easy - buy the wiring harness connector and a few inches of the wiring harness cut from a wreck at the junk yard. The ECU end is more difficult.

If you can obtain an ECU inexpensively - maybe a faulty one from a repairer - you can solder the ECU connector from that onto a piece of perf board and then the wires from the section cut from the wreck onto that.

The connections to the accessory eg. MPGuino can be soldered to the perf board or spliced into the new wire sections.

Making connections elsewhere may be easier because wiring connectors are usually generic and found in a variety of makes and models in various locations around the cars.

Making a patch harness across a wiring harness connector away from the ECU means cutting a section out of the wiring, a few inches either side of connector and including the connector, from the donor car/harness. Then unplugging the connector and joining the ends of the cut wires.

That can be done by splice, solder and heat shrink or, again, on perf board.