-c.

Headword: 
-c.
Principal English Translation: 

in, on, at, inside, over, through; in the time of (locative suffix, tells where; similar to -co)

Horacio Carochi / English: 

-c = locative suffix
Horacio Carochi, S.J., Grammar of the Mexican language with an explanation of its adverbs (1645), translated and edited with commentary by James Lockhart, UCLA Latin American Studies Volume 89 (Stanford: Stanford University Press, UCLA Latin American Center Publications, 2001), 498.

Attestations from sources in English: 

iztāc = white (iztatl 'salt')
cecēc = cold, icy (cetl 'ice')
xocōc = sour (like the xocotl fruit)
Michel Launey, An Introduction to Classical Nahuatl, translated and adapted by Christopher MacKay (Cambridge: Cambridge University Press, 2011), 112.

-c. the form assumed by the locative suffix -co after a vowel.
James Lockhart, Nahuatl as Written: Lessons in Older Written Nahuatl, with Copious Examples and Texts (Stanford: Stanford University Press and UCLA Latin American Studies, 2001), 212.

comic = in the jar
ilhuicac = in the sky, in heaven
Michel Launey, An Introduction to Classical Nahuatl, translated and adapted by Christopher MacKay (Cambridge: Cambridge University Press, 2011), 116.

Miscellaneous additional examples:
maitl = hand; mac = in the hand
tepetl = hill; tepec = on the hill or mountain
cuaitl = head; cuac = on the head

themes: 
Time, Calendar, Frequency