Not sure if this can be done with a 6vSLA. One guy tried it and he had problems with his JB3, once he switched to a 12v SLA he had no problems.
If you want to power the JB3 with a car adapter you will need to use at least 7.2V RC batteries. This is because you need at least 2v(?) difference in voltages to step it down to the proper voltage that the JB3 wants.
There is lots of info on this topic out there just do a quick search.
Later