i got a really good sequential shift light off ebay, has 4 leds, just one small unit about the size of a, errr, small usb memory stick, easy to program with different gaps between the leds, cost about £15 delivered.
it had +voltage wire, ground wire, and signal. Took my valver dash apart and mounted it where the crappy temperature sensor normally is, there is a tach signal on the back of the instrument cluster, same on the 172's i think.