I have a 02 mercury sable. I have changed the battery twice and alternator twice in 2days. I smell plastic burning and now alt is back out. What could this be?

You obviously have a wiring problem someplace, most likely in the charging system, between the power supply, alt and battery. Especially since you smell burning plastic/rubber odor. Chipmunks and especially mice love to nest in vehicle engine compartments, especially during colder months because they like the engine warmth and the seclusion and they chew on wire insulation as well as pee in the nest areas which can corrode. It could also be a lose connection or a lose spot in the wiring harness that is allowing the wires to rub against metal, breaking the wire insulation and causing a make and break connection with the movements. Try tracing the lines before you replace the battery & alt again. Please advise me how you make out.

Does a 2001 and a 2005 Mercury Sable take the same alternator?