There isn't a lot of information about the Garamantes is there? We don't know what names they used, their religion, or what their relationship was like with Egypt or the Berbers. Did they leave behind any written records?
Not that I know of, although by logic, actually they should have.
We do have some ideas of their onomastics through (scarce) Classical sources, IIRC suggestive of some Berber linguistic element (at least among the elites; there's speculation that they may be ancestral to the modern Tebu people in the same general area, who speak a Nilo-Saharan language utterly unrelated to anything Berber - this, too, is of course entirely unproven).
Their material culture however is pretty well known - before the present nastiness in Libya, their capital and other significant sites have been excavated and are decently documented - we
should have some clues about their religion from that, the shape of their temple at least. We also have their art documented by graffiti and decorated objects (IIRC, also showing traits that point to "Berber" similarities - although as always it should be noted that objects don't have language groups).
I vaguely recall having read a throwaway remark somewhere on the net about Greco-Roman influence on their architecture, but don't quote me on that. And equally vaguely, that some Classical author mentions them as worshipping Jupiter - which may mean anything, and thus means nothing except that they had something that could pass for a supreme deity to distracted Roman eyes.
Problem is, probably much this knowledge is buried in some relatively obscure archaeological campaign reports, publications behind paywalls, or simply past the tenth page of search results with obvious keys in Google.
EDIT: As I imagined, it turns out they had writing - pity we can't still read it. Also, here is a decent summary about them - I think it's where I took my impressions above from, easier to find than I thought.
http://archive.aramcoworld.com/issue/200403/libya.s.forgotten.desert.kingdom.htm