Generally speaking the option are:
FM modulators:
can be ok-ish or abysmal... the upper limit is FM sound quality, the lower limit is... interference.
BT to aux in:
Belkin AirCast Auto (or a similar product sold in the Apple store) relatively easy to install (if having stereo aux-in), decent performance for calls and music streaming.
CD changer adapters:
Have to remove Head Unit to install the plug so best do a reverse camera at the same time (if compatible with your HU)
Uses rear HU plug for external CD changers to connect to iPod/iPhone, MP3/USB or BT. Anycarlink, Xcarlink and more.
Anycarlink is one I'm also considering as they do iPhone/iPod + BT for USD $130.
Some charge iPhones, some don't, but pass-through charge adapters are readily available.
I cant find an adapter that does iPhone, MP3/usb AND BT... (but happy with the Belkin for as long as the cables hold out)
Parrot: great, but pricey if bought new and professionally installed (see above) - $600-ish or bought S/H and self installed $200 - 300 + adapter leads.
edit: fully sick how-to on the forum!
Aftermarket HU with the works: Sky's the limit!
If you have music and or radio on your bluetooth phone with the above options your car will be alternatively a mobile disco, or mobile office!
Anyway, best to google image search the HU to establish the model number, then you can check compatibility on the mfg's websites for the various adapters

edit: I mainly researched my own HU, the 6CD 6button McIntosh PF-2824I-A, aka the PF-28241... The model number is key info... but the info above is also for general use.
I'll add this smartphone/BT adapter taxonomy to the relevant pages when I get a chance...