Find a head unit (the stereo in the dash) with wireless bluetooth (best if distance allows) or a wired plug for jack that plugs into headphone jack of phone. stereo head unit should have pre-out connections to add an amplifier. connect good smooth powerful amp to some quality outdoor patio or marine speakers that have all weather box enclosure and mounting bracket such AS THESE.
expensive wakeboard boat tower speakers could work as well.
good hifi car stereo shop can set you up if afraid to tackle alone. but be ready to spend cost of quality colt .45 or talk to CRUTCHFIELD
guys if want to do it yourself.
can probably source a lot of this second hand off craigslist.
will also enjoy having the outdoor music abilities from your phone or the stereo at tailgates, around camp fire, etc. may need extra battery just for this equipment if running all night without truck engine alternator keeping things charged.
might as well add some lights to that extra battery. can get all weather switches from dune buggy shop.
i don'ta have ta cheep post