How was your EVAP leak discovered? I'm guessing here that it was likely from CA emissions testing program. Unless they specifically told you the leak was from the EVAP canister system, a faulty gas cap could also trigger an emissions failure but the cap is usually tested separately.

The EVAP system is a very simple setup. A vacuum line from the gas tank vent to the carbon canister and a second vacuum line from the canister to a small purge valve and then a line from the purge valve to the intake manifold. The electronic engine control (EEC) determines when the purge valve opens and closes to pull canister fumes into the engine manifold.

The rubber 'elbow' connections on the canister vacuum line seem to break down after years of use and develop small cracks, almost like weather checking tire rubber. Eventually leaks can develop. My '91 4.0 had this problem. A short term solution is to just coat the elbows with black silicone or find replacements.

Another source of the problem may be the purge valve. It sits under/next to the bottom of the intake manifold and is exposed to a lot of engine heat. Again, over time, the plastic nipples the vacuum lines attach to becomes brittle and can easily break away from the valve body. This is especially true if you some has been rooting around in the engine compartment, tugging on lines to see where they go. Be very careful when trying to locate the purge valve and DO NOT tug on the vacuum lines. As one who committed this sin, you don't want to have to purchase a new valve just because one small nipple broke loose. There is no easy fix.

The vacuum line running from the canister body to the purge valve runs past heater hoses and a radiator hose. It is possible that a misrouted hose has rubbed a hole in the vacuum line. Since you have tried blowing into this line and find it "blocked" would tend to indicate there is no leak in this line. The blockage is the purge valve in a closed state.

There are diagnostic codes for the EVAP system. If you do not have these, most parts stores can pull the codes for you for free. Your model year likely uses OBD-1 codes, and will return both a 2 and 3 digit code. The two digit just specifies which major system is problematic while the 3 digit is more specific as to which component is faulty. A specific code number will greatly help you pin down the problem area.

Perhaps one of the posting gurus here can find a schematic for you to see the location and visual of what the various components look like.