Google and alternate pages

I'm managing a web site for a smal business selling car parts --
specifically air scoop grilles. And I'm having Google problems because
of the spelling of the word grille.

People in different countries spell the word differently -- either as
grille or grill. So, to maximize search results, I created two seperate
product sections with different spellings, and cross-linked them, so
Google would find both.

The result: searching for the phrase "custom mustang grilles" has
dropped the site's ranking to the mid-20s from where it used to be in
the top five.

Meanwhile, searching for "custom mustang grills" does place the site in
the top ten, but the product pages are not being indexed as they used
to.

The site is here: <http://ccmustang.com/>

The cross-linking is done in a manner that is invisible to the eye -- I
am using two transparent GIFs named grills.gif and grilles.gif. I know
that Google penalizes white text on a white background, but I kinda
doubt it is able to discern that grilles.gif is completely transparent.
Yet, what would account for the drop in Google ranking?

Any help is appreciated.

--
Jim Royal
"Understanding is a three-edged sword"
http://JimRoyal.com