PV junction boxes. Commonly found on the backs of solar panels, but you can buy them separately. I used one on my roof, and one on my rear hatch.
Drill a hole in your roof/hatch just large enough for your wires, but small enough to be covered by the junction box. Use a sealant on the the base of the junction box, and I added a couple screws to help hold the boxes in place, just for good measure. Remove the diode and metal tabs that come inside the box, and just make your wire connection in there. The lids have little silicon gaskets to keep water out, and o-rings to seal around your wires that exit the box and go to the solar panel.
I was looking for a picture of my installation, but the boxes are hidden under the solar panels. I believe these are the ones that I used: http://www.ebay.com/itm/PV-SOLAR-PANEL- ... 2570f0132d