staining the back seats is when you engage in sexual activityin the back seats of a car. In most cases, stains are left from the sexual activity the occurred in the back seats, hence the name, "stain" the back seats.
Chris asked Stacy if she wanted to stain the back seats with him and she rejected him and called him a pervert.
This phrase is used to respond when one person says something you disbelieve, and then another person comes in to support them. The point is that a lie doesn't become more true simply because another person (of similar sketchiness) tries to back it up.
A: No way man, I wasn't even therethat day, you can even ask Jimmy, he'll tell you I wasn't there.
B: Jimmy? Oh, awesome, now Jimmy's backing up your story. Well, just goes to show, "one lies and the other swears to it."
When one has spent the night before on Drugs / Drink , (or simply turned the room temperature to tropical) so upon waking feels a certain moistness in the sheets - Have you urinated , no , you've sweat the bed.