When l hit the temp button, it just flashes -35 deg. Anyone know how to get it working?

This can happen if the sensor has gone bad or if it has become disconnected. Did u just buy this car, or have u had it for a while & it just started happening? A front end collision can cause this. Some shops do not hook it back up or maybe they do not replace it.
The sensor is in one of the brake cooling ducts up front, or behind a fog light if u have those.
l do not have an E36 but look at this page for more info http://bmwe36blog.com/2007/07/04/faulty-temperature-sensor-in-bmw-e36/

l have got an E36.maybe something is just blocking the temperature sensors themselves.take a look..or maybe.it is just faulty.like many E36 is out there..you better have a technician look at it.not BMW themselves, l do not think they fix the E36 anymore.it seems to be a huge problem for the E36