Trunk is a good location for several reasons.
2. If you have amps or other devices
3. plenty of room to work
4. hard to steal what you cant see
5. wires are not exposed
6. Never had an issue with heat or cold.
7. Plenty of room for expansion.
I think it really ends up being what you drive. The carputer for my protege is in the trunk while the carputer in the 350z is in the cabin in a cubby hole behind the passenger seat. That box is very small but has no issues as well.