I know ive asked this before, but I bought a vdo tach from postons about 7 months ago and hook it up as per the instructions . but it still does nt work. called vdo and asked about a 5-volt sq.wave mod but they act like they dont know what im talking about. called postons and they said they have sold non-modded tach before but no longer sell tachs how do I FIX THIS. THANKS