where is the computer diagnostic port?

Hello,

I am assuming you meant to put a 1989 Audi 90. They did not make the 1990 in the 1999.

The diagnostics port can be either of two locations, depending on the manufacturers date for this vehicle.

I can be under the hood, in the fuse box that is close to the firewall.
Or it can be under the dash next to where the steering shaft goes into the firewall.

Look for 3 connectors next to each other.

Here is a chart to VW OBD1 codes, they are the same as Audi codes.

https://www.2carpros.com/articles/volkswagen-code-retrieval-procedure-and-code-definitions-1988-1995-odb1

Please let me know of any questions.
Thank you.