I just returned from Chitwa Chitwa and we rented a car and used GPS. Here's what we did:
We rented an SUV from thrifty at JNB via Expedia. I don't remember the price but they were having a deal on the SUV so that it ended up being the same price as a compact so we figured we couldn't lose. We did NOT rent a GPS - I'll get to that later.
Chitwa Chitwa has a map on their website with the suggested route from JNB and I suggest you go that way. We deviated slightly and the roads we ended up on, while paved, were in such poor condition with potholes and many areas of construction that we didn't save any time despite them being the shorter route. I had an email from them giving me detailed directions but I seem to have deleted it. I'm sure they'll send it to you as well - they will be emailing you asking when and how you plan to arrive. When you tell them you're driving they will send you the same email, I'm sure.
As they will tell you, the only way to Chitwa Chitwa is via the Gowrie gate. We found the gate no problem with their directions + our GPS. The roads after entering are dirt jeep tracks and they are bad in places. (At the time we were there, anyway.) We were glad for the SUV. It could have been driven in a regular car but certain areas were quite bad and steep and so we felt we made the right decision getting the larger car.
As for GPS - I have an iphone, and downloaded the Sygic app for South Africa and used it for our entire vacation. The great thing about it is you DO NOT have to be on a network in order to use it. You do need to have your phone ON (not in airplane mode) so it can use the phone's internal GPS, but you will not incur any charges. It really is pretty amazing. It was useful because, even though we were also using a map, it was nice to have the KM to dest, alternate routes, POIs (we used those a lot) and it also alerts you where the traffic cameras are. It even knows when you're speeding! So if you have an iphone or an ipad that is network enabled then I highly suggest it. It cost $39.99 but was worth every penny. And again, i stress you DON'T need to be on the network. It was like magic. :)
Chitwa Chitwa warned us not to use GPS to get to them, I guess because most GPS have you go to the wrong gate. So we used ours to get to major towns on the route, all the way to the gowrie gate. (The gate itself was not in Sygic's system, which uses TomTom maps.) Once inside the gate the signs to Chitwa chitwa are clear.
That said...to answer your car charger question. YES, it works. And if you use Sygic, you'll need it. The only bad thing about Sygic is it's a drain on the battery since it's constantly locating your position. So I had my phone plugged in to the car charger (which I brought from home) almost all the time just to keep my battery charged. It did not fry my battery. We also used it to charge my husband's iphone 4 (i have the 5) and our iPad.
Edited: 12:28 pm, March 08, 2013