Techniques - Soldering wires
Soldering engine management system wires requires a good clean soldered joint, as the currents and voltages may be quite low
Shortening a long loom, maybe best to leave original connection to terminating pins/plugs and cut/join the wires a good few inches back from the original connection, to avoid damaging the termination or its insulation - these are sometimes small, corroded locally or imperfect, so the elss messing the better!
A good single solder joint some few inches away from each termination should be fine, don't twist them together, pre-heat and tin each bared length, then lay them flat against each other (overlapped), run the iron along them, don't move a muscle for 2 or 3 seconds until it flashes off, insulate and job done. Slide the shrink insulation on before you lay each one up. Don't dally with the iron or solder gun once both have flashed together, look for a quick puff of flux wafting up - that's hot enough. Don't use an iron that's too low power, I'd say 20W minimum, a 25W ~ 40W iron used quickly is much better than heating a foot up either side up with a silly 15W one while it gets the joint up to temp. Too small an iron is a certain recipe for cold-soldered joints! 1, 2, max 3 seconds it should be flowing molten, remove iron as a waft of flux smoke goes up, hold together, don't move, eet's done! Move on , do next one. If any of the wires look corroded when you strip them back (about 1/2" minimum) or don't tin up easily, then you may need to wipe with wire wool first (fine 00, or 000), can even finish tinning with it when wire is hot, plenty burnt fingers though
Other methods to be advised if you have trouble soldering. A bulldog clip mounted on a bench or in a vice, or on a block of wood that can be moved around is handy. Hold iron underneath difficult dirty ones, and rub solder along the top forcing the heat to come up through bared wire.