I'm guessing that it either 1) connects directly to the battery, in which case you'd have to rig up a way to attach a cigarette ligter plug to it in order to use a charger, or 2) it plugs
into the car's cigarette lighter plug. If that is that case, then you might need a single to dual adapter, so that you could plug both the charger for the phone and hte solar charger into your cigarette lighter plug single to dual adapter, plug the adapter to the car's cigarette ligher, and charge both.
in hte latter case, the single to dual adapter should *also* suffice for use in hte field as the phone's car charger will automatically regulate both voltage and amperage (by design).
Also, technicaly specifications show that microUSB is supposed to provide up to 500 mA, but most car chargers vary up to 1000 mA, and the electrical chargers rom Motorola supply 800 mA. The lower than 1000 mA that is supplied to the phoine, obviously the slower the charge. The quality of the product (wires, charger, etc.) also makes a huge difference. I bought some USB-A to microUSB cables from monoprice - they are much more sturdy, thicker, better insulated, and larger guage wire inside - and they, from the same electical outlet device made by Motorola charge my phone at different rates. tested with two different batteries....
See my post here for the monoprice links to cigarette lighter adapters
http://androidforums.com/motorola-droid/17365-droid-battery-issues-charging-slowly.html#post110803
in fact, that whole thread is a very good read....