[Mee-n-Mac]

Not knowing what you have for a system it's hard to be sure. Like Skip said, pressure washing the speakers is not a good idea. If one or more can be easily removed you can check them on a home system. If practical you can also substitute in an alternate speaker to see if the head unit and amp are good or bad. If you can't easily remove any of the speakers, I'd ask if you have a 4 speaker system. It's unlikely that all 4 speakers would have been washed into oblivion. No sound from all of them would make me wonder if they are plugged in.

You have either a combined head unit/amp (likely) or a separate head (AM/FM/CD) unit and amplifier. If it's the former we know you have power and ground but perhaps the speaker connections aren't right. Check these first. If you have separates then in addition to making sure the co-ax cables that connect the head to amp are plugged in, make sure the wire (often blue or orange) that "tells" the amp the head unit has been turned on, is also connected. Without seeing it (or knowing the model number), it's hard to instruct you with any more detail.