I had read that Mercedes released an in line amplifier to deal with low volume problems with their ipod integration kit. http://www.mercupgrades.com/iPod+Vol...rticle-45.html The part number is A204 870 39 94. I have found several discussion threads about this product, but most have used it in mercedes other than the SLK.
I called my dealer and he went ahead and ordered the part. Today I brought the car in for it to be installed, but he told me that they checked with Mercedes and that this is not the correct part for SLK models. Apparently there is another similar product for the SLK and he has apparently ordered it for me (I do not know the part number). I have reviewed all the threads I can find about this part number, and I can not find any reference to a similar but different part for the SLK. Does anyone have any further info about this. Dan are you able to comment on the part number for the SLK?
I've used the 204 part on an SLK before now, some of the newer iPod modules also have a software option built into then to turn up the volume without needing this amp and you set it via the cluster.
Thanks for your response Dan, mine does have the newer software version which allows the volume to be increased, however even at the high volume setting, the volume is still much lower than that for the radio and CD player. Is it true that Merecedes does not recommend using the part number I listed in the SLK?
Indeed the dealer is doing the install. I was just surprised to hear that Mercedes had another part number for this in line amp. I must say I am suspicious that in the end I will be told that no other part is available and that part A204 870 39 94 is not meant for the SLK. I believe others have reported this happenning , yet some have been successful in having this part installed. At the moment though, the dealer tells me the "new" part is on order.
I don't have an iPod (and probably never will), but I think I remember reading when this problem has come up in the past, that the iPod has something like "line out" or something else somewhere that puts out a signal that the MB iPod connection does better with. Try poking around in the iPod and see if you find some other "non-amplified" output.
Alternatively, do a search here and in other MB forums because this has been discussed several times before and I believe resolved with nothing extra to purchase.
Ipod low volume solution 
