[Bobboloo]

I assume that 89s' have electric speedos?

Anyway, it could just be a dirty connection. Either at the back of the speedo or where it hooks up to the sensor located at the...distributor? I'm guessing where the sensor would be because my car has a mechanical speedo so maybe someone else with an electric speedo can chime in on it's location.

That it starts to work after 15 or 20 seconds suggest that maybe its heat related. So you might check in the engine compartment first as opposed to the speedo.

What happens to electrical connectors over time is they corrode a little. When the parts heat up they swell a little and close the gap,that was created by corrosion, thereby completing the connection.

So to remedy just pull the connector off and clean it and replace. Use of some di-electric grease wouldn't hurt either.

Good Luck
Bobby