Along with cost, the second biggest challenge at the base of the pyramid markets is building "trust" in the community, the local equivalent of building a brand.
First, find out which media people trust the most and aim to be on three of them, the highest ranked you can afford. For TV and Radio are the most respected across the BoP world but few companies choosing to address them can afford that kind of advertising.
Second, find out how information flows by word of mouth in the community or region you seek to serve. There may be rural and urban differences, for example, in a "static" community like the factory shopfloor your maintenance men are the "movers of knowledge" or infomediaries, but in a village or market, it would be the one with a fixed location yet highly central, like the local paanwallah, Chotu.
Third, enlist the support of these individuals, giving them special status in the community, perhaps, look at how this NGO which is sharing information on nutrition and health is organizing their infomediaries.
Further reference can be found here, on the Role of Mobile Infomediaries among BoP and of course, the brand aspect is quite well documented already.