As long as it doesn't dry out then it should be OK - rots the arse out of your car and makes it stink though.
My concern would be the insurance though - if the stuff gets nicked from your car and its been there for a week will your dive kit insurance cover it or will your car insurance? Leave it an unreasonable time and if it does get nicked and you do try to claim on your insurance it might be one of the ways they'll use to avoid paying out.
The advantage of stupidity over intelligence is that stupidity has no limits.
T-7
Consistently lowering standards in diving since 1985.