Broken Sensor Sterling Touch Immobiliser

There are 4 wires going to the touch pad unit which has an LED in it. The unit has 4 pins (or legs) to which the wires must be soldered. As you say, there are two blue wires plus a red and also a black - although as you also correctly observe, only three wires are present (blue, red and black) in the connector at the other end of the loom which plugs into the control module.

The blue simply splits two ways within the loom, and these are earths. The black carries a pulsing voltage to drive the LED, and the red (which should measure a constant 5v on a voltmeter) is an input which senses when a key is touched on the pad.

Use a continuity tester or ohmmeter to establish which of the four pins on the touch unit connects to the CENTRE pin of the touchpad. Solder the RED wire to this pin. (Best to start by putting some heat shrink up the wire as the pins on the touch unit are quite close together and there's not much to stop them touching.)

Again using the continuity tester (or ohmmeter) establish which of the remaining three pins goes to the OUTER ring of the touchpad. To this, solder one of the BLUE wires - it doesn't matter which one. You must however get the wires to the touchpad the correct way around in order for the key to be read.

You're now left with one BLACK and one BLUE wire which drive the LED, and two pins on the unit which have no connections. The LED will only light when you get these the correct way around. You will not do no harm if you connect them the wrong way round - the LED will simply remain unlit.

One you have soldered the BLUE and the BLACK wires in place so the LED is flashing, simply test your work by touching a key on the touchpad. If all is well, the LED will light continuously for a period and it should be possible to start the car.