Pepsi Light was the lemon/cola (these were the two flavors available) flavored Pepsi sold in the 70s and 80s that was discontinued afterwards. Nowadays this is the name under which Diet Pepsi is distributed outside English speaking countries. Having known about the product, deciphering the ad would be pretty simple now; I guess. :-)
Guys, do drop in a few words about your opinion on the ads posted in adMad in the comments section.