where is the o2 sensor on a 1988 cabriolet

In the exhaust stream somewhere. Some cars have one in the cat converter ....follow the exhaust from manifold to muffler/cat, it will be there somewhere,

'bout $80

Some exhaust systems have as many as three. All can be measured with a multi meter if you know what the readings should be you can even identify a bad one before you swap out a good one with a new one. Might be harder to change than it looks because all that heat will shrink it into place, rust, etc. May need torch, six pointed socket, penetrating oil to do job.

Some cars can have four. V6 & V8 engine can have two on each side. Basically most engines have what's called a up stream sensor that sends feedback to the ECU to regulate the air/fuel mix. They also have a down stream sensor that monitors the health of the catalytic converter. Sometimes the lower O2 sensor is used in fuel monitoring too.

I know all the general answers, I want know SPECIFICALLY where IT IS ON THE 1988 VW CABRIOLET/GOLF.

Assuming there is only one, it is on the exhaust system. If you can't at least locate it by yourself, you won't have a chance of changing it any way. Take it in and save further frustration.

Open the hood. Look at the exhaust pipes. At the junction where the four come together, you will see the sensor. Couple inches long with wires coming out the end. It needs a special socket to remove and install.