Hilux pre-04 behind back seat plug holes

Hi

I've been shopping for a 2nd hand dual cab, have been finding that when you pull the back seat rest forward, at each corner showing up straight away or sometimes under the rubber floor covering corner, are two plastic plugs. Open them up, all it takes is a fingernail, and I find evidence of exactly where it's been. Sometimes the tiny well is 1cm deep in red dust, sometimes caked mud, sometimes sand.

Point is, other than to let people like me know something of a lux's history, what is this plug for and does it lead straight to the running gear?

thanks