You can buy a plastic bracket to get 6 1/2 to fit in the larger encloser, or you could buy the 6 3/4 speakers. Many companies make the bigger speakers. Check out www.crutchfield.com and look at the speaker sthey have, just to give you an idea.
Any decent stereo shop should have the brackets, or at least could get them for you. Crutchfield sells the brackets for under $12, so you could just order them from there.
Also check out, www.thezeb.com since they have an assortment as well. I don't know about a 1994 model but with my 2001 some 6.75 won't fit in the door. Also, the Blaupunkt 6.75 are too large for the rear deck on my Altima too.
The 6.75's are there but you have to dig a little for them.
If worse comes to worse, you can buy the brackets and install 6.5's.
I just installed some 6 3/4s in the rear deck, RF FRC1206...they sound pretty good...I would recomend them...and they are onsale in crutch for like 60 cuz the new models are coming out..but ive seen them for as low as 50. Here is a link to crutch's listing of 6 3/4's...take a look at one you like...then check zeb or sounddomain for prices...
(not sure if that link will work but try it, or go to crutchfield.com, car audio, 6 3/4 speakers)
ONE THING I WOULD RECOMMEND LOOKING AT THOUGH!
Make sure the top mount depth of the speaker isnt more than 2-5/8" !!!!!!!!!!!! That is the specs for the RF's i bought and they just BARELY fit under the trunk bar...so....also look at that spec when buying some..you can do some modifying to make other speakers work im sure, but if you want easy fit...thats the largest depth I would go......GOOD LUCK!
Most American-made speakers are oversized. If you went with something like a Boston Acoustics Rally series, Jl Audio XR, or Diamond Audio Motorsport or Hex, you could get a perfect fit and have incredible sound.