If i remember correctly it in the low side just out side the dryer. Youll see a gold colored connector where the 2 lines are different sizes. Open up that connection and the orfice tube will be in the smaller of the 2 lines. Be sure to properly recover the r134 and have it recharged professionally. If you have the system open for longer than 10-15 mins or live in a high humidity area and mabe just because its an older truck you should replace the dryer at this time too. It will only help in your quest for a cold a/c system. Also if you open it all up and you pull the old orfice tube out with some needle nose plyers and you notice small aluminum peaces in the tube you have a problem. Your compressor took a ****. At that point its good to flush all lines individually, including the condensor. Replace the compressore, orfice tube, dryer, and again have it charged also i always add alittle bit of die in the system just incase later on i get a leak it makes it alot easyer to diag. Oh and DONT forget to install the correct amount of oil, and make sure its the right weight of PAG oil too. I hope im not to late, lol.