1999 Plymouth Voyager Changing spark plugs

There are several methods that can help but nothing is going to make it easy. The removal process is almost totally by feel becaus of limited clearance.
Some access can be gained by removing a few components on the passenger side of the engine closest to the firewall. (Brackets) Older models had plenty of room from underneath if you follow the exhaust up to the engine, but this year is quite different. Still, support the vehicle sturdily and take a look.

Good Luck.