In the front of the car is the condenser, and there is a receiver drier with a sight glass on the top where the lines enter the condenser. I would look at the sight glass after adding the specified amount of oil to the compressor, and add 3 additional ounces to the compressor for the rear air conditioning unit. Charge the system on the low side until you see all the bubbles in the sight glass disappear. Sometimes you will see a fine thread of refrigerant moving through the site glass, and that is normal. If your cars original system was charged with R12, and you are converting it to R134, there are additional items that need to be removed and changed to accommodate the R134 in a R12 system. I have never done a conversion, and I can't advise you exactly what changes will be necessary. 1992 was the year that R134 was introduced, but not all car manufacturers made the change from R12. You need to have a factory shop manual, and start by using the amount of refrigerant for the basic car, and then adjust up from there.