I was curious, too, and found this:

http://cseligman.com/text/planets/magnetism.htm

The author estimates Mars' core temperature to be 5000-7000F, compared to an estimate of around 14000F for Earth. Obviously, the planets' respective compositions are different, so I'm not sure if one could just scale a temperature estimate linearly---but what the heck, this is the Internet and no one's grading me, right? ;)

So the core temperature ratio is 5/14, and the size ratio is 1/3, so:

  (5/14) * (1/3) * 44F = ~5F

That comports with the current consensus regarding subsurface water, because scientists think that it's likely frozen or perhaps a brine (which would depress the solution's freezing point below that of pure water).

Tunneling is probably a good idea anyway, because the surrounding rock will protect and insulate the habitat even if the subsurface temperatures aren't enough to warm it.

Edited: Oh for God's sake, I confused degrees Fahrenheit with degrees Celsius.

For long term stays we'd need to dig underground dwellings on Mars anyways, though not nearly as deep, as the radiation that gets through its atmosphere will be very harmful to anyone living on the surface for multi year time spans.