if you want to play ipod there is few ways to go with it.
1)AUx - in
2) Ipod interface
3)fm transmitter.

Aux-in and ipod interface gives the same quality as you listen through your headphones while fm transmitter might sound abit flat. even the good ones like belkin and kensington doesnt really gives you the best juice from the price you pay.

Aux-in is what most people do but you have to control your tracks through ipod and not the HU. if interface you can control ur ipod through your HU, and price will be more expensive(around rm500-800). fm transmitter might just end you with the cheapest rm30-40 to rm300 for branded ones. Alpine ex-10 might cost way more... mostly RM1k plus.

if mp4/mp3 players, you can use fm transmitter with 3.5" audio jack or just aux in will do. or better still, if you HU support usb, you can insert it too since mp3 player nowadays are used as mass storage.