It really depends.
Was the dish cooked before? If it was, and is a dry food, or is a liquidy food and is still warm (This is the Rema's opinion, the Mechaber requires it to be yad soledes), there is no problem of cooking on a biblical level. If it's a cool liquid, there may be a biblical prohibition (See chazon ish who assumes that Rema concedes that this is only a rabbinic prohibition...).
If there is no biblical problem, the only possible issur is chazarah, a rabbinic prohibition. This issur does not apply to places where one does not normally cook (See OC 253:5), such as on a radiator. Therefore, where it is halachically called previously cooked, it is permissible.
If it was not cooked, or is a liquid which has cooled off, then the biblical prohibition of cooking applies, provided that it will be heated to Yad Soledes Bo. If it will not be, then it is permissible (OC 318:14). The definition of yad soledes in terms of degrees is the matter of much debate (on which I'm sure you can find a lot of literature online), which I don't really want to go into....
If one already cooked the food in a prohibited manner, the food may be prohibited to consume. This depends on if the prohibition is unanimous (MB 318:2).
As always, be sure to ask your LOR.
I apologize if not everything is sourced. All of the stuff on biblical cooking can be found pretty easily in Siman 318, and on chazarah in Siman 253).