Expect to strip any wax off. Use a typical dish detergent, spray (not wipe) it on, let it sit a long time to soften up the grime. Reapply if it starts drying out.
After it has sat a couple hours, a pressure washer on a very low setting or even a plain old hose and sprayer end should be used to flush away as much as possible, only as much pressure as needed before the final step of using abrasive wiping with a soft cloth. When wiping, it helps to have a stream of water flowing on it to do more of the work rather than only wiping then rinsing.
You'll probably want to clay bar it next then put a sealant on.