There are many many ways you can do this now a days. There is buying a amp with built in BT and just using that through your phone or tablet.
There is running a cable from any amp to a BT module that will pair with your devices that way and have a small volume knob you can mount somewhere for volume and change track.
There is running a device through the headphone jack to a Y splitter into your amp but you have to be very careful about adjusting your gains and how high you turn your volume up on your device.
I have included a few links to get you started.
RE Audio BT900.4 4-Chan Class AB Bluetooth Car Amplifier (BT9004)
Cadence XAM500.4BT 1000W Bluetooth Smart Ready 4-Channel Xenith (XAM) Series Class D Full Range Amplifier
NVX MVPA4_XUBT3 400W RMS 4-Channel Bridgeable Micro Class D
JBL GTR-104 Stadium Series Bluetooth Car Amplifier with Clari-Fi
Kicker PXiBT50.2 PXi 50W 2-Channel Bluetooth Amplifier + $50.00 Sonic Gift Card
JL Audio MBT-RX (mbtrx) Universal Marine Bluetooth Receiver
Wet Sounds WW-BT VC (wwbtvc) Bluetooth Receiver Module with
One last option if you want a LOT of expand ability and EQ adjustments. Look into buying a sound processor with the BT adapter like the RF 3.sixty
3SIXTY.3 - 8-Channel Interactive Signal Processor w/ 248 Band Parametric EQ | Rockford Fosgate®