To charge the AC on a 2006 5.3L express van you locate the AC unit under the hood. The owners manual will have diagrams on the engine parts and the type of coolant needed to recharge the AC. You follow the instructions on the coolant.
I have a 2003 Grand Prix GT and the AC recharge port is right behind the motor on the firewall. Look for the silver pipes coming out of the firewall right near the middle. You will see the recharge port on one of the pipes there.
The 1990 Mitsubishi does not have an air conditioning recharge line. The vehicle has an air conditioning recharge port. You can recharge your air conditioning system through the low pressure port on the air conditioning compressor.
You can recharge your vehicle ac system through the low side port, you can also check the pressure through the same port. There is a website with helpful information about your vehicles ac system www.idqusa.com.