In order to play music from an iPhone in an 98 Acura RL?

What will you need

Get what is called an FM Modulator (sometimes called an FM transmitter). These devices connect to a standard 4mm headphone jack and transmit a signal that can be picked up by your FM radio.

A word of caution. The quality of these devices vary widely.

I have owned a couple of these devices over the years. The first one I got was made by Scosche. It worked reasonably well. But it had the limitation of only being able to broadcast in the 88 MHz frequency range. If I'm in a location that has an FM radio station broadcasting near this range the FM radio station will overpower my FM transmitter.

I also owned a Belkin. It could be set to broadcast over any standard FM radio frequency. But this FM transmitter was a complete POS. The signal it would broadcast was way too weak. It sounded like a radio station that you were just in range. I rarely used it and eventually tossed it into the trash.

Scosche now makes a model that pigtails between the vehicles radio antenna and radio. I would suspect that this one would work pretty well. Reviews vary however I think this had more to do with improper installation. The antenna connector on an automotive radio is not standard. I bet the people having problems with this device should have used an antenna adapter.

If I ever decide to replace my old Scosche FM transmitter I plan on getting the Scosche model FMMOD02. Scosche sells them for about $50 but I have seen them on several websites for about $30 online.

Disclaimer. I have nothing to do with the company that makes this transmitter or any place that sells them. Just giving some friendly advice.

Stay away from FM transmitter or modulator, the sound quality from the portable devices are horrible, worse in your Acura because part of your antenna is on the rear glass

for the best sound quality buy an auxiliary audio adapter for Acura radios, this plugs into the back of the radio and allow you to input audio from any portable source iPod mp3 dvd
http://www.oemautosound.com/p-8-pie-acura-honda-auxiliary-audio-input-1992-2001-hon-aux.aspx

you can also use an iPhone interface to control the iPod form the radio