spark plugs

Is there an easy way to change the back sparkplugs on the 99 windstar 3.8 engine.

I have a '96, which is somewhat different....
I have easily changed my rear plugs by driving the front up onto car ramps.
Put in PARK, Set the Brake, and put a block behind one of the rear wheels.
Let the engine cool, plugs come out much easier, and the catalytic converters will be cool.
Slide in under the front, way back, far enough that you are looking up the front of the firewall. If you went back any farther, you would be looking up into the floor pan.
At this point, you will be able to see up to the rear 3 plugs. You can reach up and get to the plugs.

Others have removed the cowling for the windshield wipers, and gotten to them from up top, with great success.

Make sure to replace with double platinum spark plugs.
Double platinumn means that both the center and ground electrodes are platinumn enhanced. The Bosch spark plugs are NOT double platinumn.
The only brands that I have seen that are double platinumn are the Motorcraft and Autolite brands.

That's how I changed the rear plugs on my '01 (from the underside). I think the got the right-most rear plug from the gap in the fenderwell. I used Autolite double plat plugs.

I get all of them from the top on '96, '98, and '01 models. It takes a bit of contorting and the right combination of extensions, but it can be done. Due to the angles, they seem to be easier to get from the left (driver) side.