Soldering isn't used OEM due to time factors for assembly, and because it actually is more brittle than a crimped connector. It leads to failures during durability testing, specifically during the vibration cycles.
In general, design temps for the vehicle are:
General Passenger Cabin: 85 C (185 F)
Cabin Headliner and other ares with direct sun contact: 105 C (221 F)
Underhood: 125 C (257 F)
At least, those are numbers for Chrysler. Other OEMs(especially Japanese) sometimes have some variations, although underhood is generally 125 for everyone.
Generally, you are worried about:
Jacket heat resistance
Jacket material resistance (such as oils, solvents, etc)
Number of strands.
All in all, its not really common for people have failures of the wire itself, except for melting wires on something hot like the exhaust manifold.
Sticking with a nice stranded good size wire, you should be fine with the jacket types above.
14 gage for 15 amp
12 gage for 20 amp
10 gage for 30 amp
You can get away with a lot more current in specific situations, but those are nice safe numbers. When we are determining our harness sizes for the vehicle in general, we are considering the max current draw under worst base conditions, fuse size, length of run, and how the wire is bundled (capacity cahnged based on whether its indivitual or buried in a larger harness, due to thermal considerations).
Generally, OEM wire sizes are way to small for my taste. But, being on the 'inside', I understand all the considerations into harness design, so I know why they do what they do, even if I don't like it..