Mewseee

Hey everyone! Ive got a 1990 B6 turbo with a diypnp. Everything is working perfectly but ive decided to go with a virtual dashboard, ive got everything sorted except speedo. Ive worked out that the factory dash from an auto has a speed sensor in the form of a magnetic reed switch that closes to ground with a 5v pulse, my problem is i cant find a way to get the ecu to interpret the 5v pulse. Does anyone have a simple solution that ive over looked or is it possible to use the onboard lm1815 circuit?

Thanks Mewse.