DIY simple line out converter for "floating ground" car head unit?
It depends on the type of output your "head unit" has what solution you can use here.
Are you sure it's not just a single ended push-pull output? What does the manual say exactly?  

Assuming it's a truly balanced output (i.e impedance matched) then it may be transformer coupled, or actively balanced using two driver ICs or single ended with impedance matching from pin 3 ("cold") to ground.
If it's just a "quasi" or "pseudo" balanced (made by just adding an inverted output but not impedance matched) then you must be a bit careful and not connect the cold side to ground.        

The connections for the transformer balanced situation would be hot wire (XLR pin 2) to the unbalanced input, and cold (XLR pin 3) to unbalanced ground (you can connect pin 1 to pin 3 for cable shield)
This method can also be used in the case of a truly balanced impedance matched active output (shown in bottom picture as "Typical Pro Output"), but do not connect the equipment grounds together (don't connect shield at both ends)
Here is an example of a transformer balanced output to unbalanced input connection: 



For a standard push-pull output, connect pin 2 to unbalanced input, pin 1 to input ground and leave pin 3 floating, do not attach it to ground (with a standard push-pull output it may cause overheating of the drive IC)
You will still get the same signal, it will just be half the amplitude of the balanced signal, which is fine since you want to cut the level in any case:   



The best method for interfacing a balanced/push-pull active output to unbalanced input is to use a transformer, preferably with the winding ratio to attenuate as desired:



The above methods can be used with a suitable divider for level matching. Make the divider output impedance around 1/10th or lower of the input impedance so as not to load the divider too much.  

You will get widely ranging views on the best way to interconnect balanced and unbalanced equipment (especially about how to connect the shield wire), so in order to avoid confusion try and understand what is happening, why certain approaches might not be a good idea and what to try if you get problems.
When testing at first, check with a scope/multimeter to make sure you have not shorted anything to ground that shouldn't be (check current flow and voltage levels - you can also use an appropriate fuse to protect outputs if they don't already have one present)     

Here are a couple of decent links to get you started:

Interconnection of Balanced and Unbalanced Equipment
Balanced and Unbalanced - How to do it right
ESP Audio Pages (same site linked to in Danny Kmack's answer - from what I've seen of it it's an excellent source of reliable information)