The EVAP canister on your 2003 Chevrolet Silverado 1500 should be located near the fuel tank. Either in front, behind, or on top of the fuel tank.
I am assuming your 1998 Chevrolet 1500 Pickup has an automatic transmission. There should be a dipstick located near the fire wall on the passenger side. Check the fluid level with with the transmission fluid warm and add fluid via the dipstick tube as necessary.
You are correct, That hole is where the distributor used to be in that engine block. The ignition system in your 1995 Buick Century does not use a distributor. What is in the hole however is not a plug, it is a gear driven shaft that drives the oil pump just as the distributor shaft used to do. There is an o-ring at the top - just like a distributor o-ring. The o-ring is probably hard as a rock and not sealing any more. The oil pump drive will need to be removed and the o-ring replaced. Even with everything out of the way it can be a bit difficult to remove the oil pump drive. If you find you need the help of a repair shop please use the following link to find one near you - http://repairpal.com/directory?address=46227&car_brand_names=Buick
I sounds like the battery is dead or perhaps there is corrosion on the battery terminals. If you cannot check this yourself please use the following link to find a repair shop near you - http://repairpal.com/directory?address=89141&car_brand_names=Toyota
My service information shows that the instrument panel must be partially removed in order to access the heater core in your 1996 GMC Sanana 1500. I would recommend you have a local repair shop take care of this for you. Please use the first link to find a cost estimate and the second to find a repair shop near you - http://repairpal.com/estimator?car_type_id=6424. http://repairpal.com/directory?address=01013&car_brand_names=GMC
Once the fault has been corrected which caused your service engine soon light to come on in the first place you can do one of two things. Reset the light using a code reader or scan tool. Technicians will normally do this after completing a repair, your local auto parts store may also perform this service. The second way is to wait for the on board powertrain control module complete its system checks and turn the light off itself. This could take days or weeks depending on what the driving conditions were when the fault code as first set.
I have seen this problem on some 2002-2003 GMC Yukons, so this may or may not apply to your vehicle. What I have seen is the load of the compressor causing the belt and tensioner to jump, hitting the transmission cooler lines causing a rattling noise. There is a service bulletin from GM for this issue #03-01-38-019A. But again, it does not specifically apply to the 2001 model year. The repair is to replace the compressor with an updated unit and install an inline filter. If you can verify you have the exact symptoms as I have described you may still have an issue with your compressor. Perhaps discussing this issue with your local GM parts supplier would be a good idea. They may be of help in determining it the 2001 and 2002 compressors are the same, if so perhaps you could try the updated compressor in your Yukon.
If the A/C system is low on refrigerant the system will automatically switch over to econ. I would recommend you have the A/C system in your 1996 Cadillac DeVille inspected for proper refrigerant charge. If it is low on charge perhaps leak diagnoses should be performed.
If trying your spare key does not work, chances are the fault is with the Passlock sensor. For more information please read the RepairPal article on GM Passlock anti-theft systems, here is the link - http://repairpal.com/gm-passlock-anti-theft-systems.
The cooling system in your 2002 Jeep Liberty should be check for a leak. Special tools are available to pressurize the cooling system. While the system is under pressure the cooling system should be completely checked for leaks. If no leaks are noted and the coolant loss continues it could be possible that coolant is leaking into one or more of the cylinders.