Bala the ASS would have left open some gaps in firewall when your car met accident earlier. Remove the sound deadning carpet from firewall in engine bay and see for any hole there. There should be gromets on every pipe/wire that goes in and out with no space visible. Only place that I know through which rats can enter inside cabin is through fresh air opening in boot, install wiremesh there. Rat's cant enter through fresh air intake opening for AC as there is AC filter below that and to enter cabin the rat would have to chew AC filter first even than he would get stuck in blower and most probably die when you start AC blower.