Theres a lot of chat on the forums about changing the head unit on a P38 and various issues that it seems to cause, so i thought i'd post my own experience.
My car has the "high level" sound system with three speakers and an amplifier in each door.
I really wanted to add bluetooth for handsfree, and i had a spare Pioneer bluetooth headunit i'd removd from an old car, so ideally i would fit that and job done.
reading online all sorts of stuff, i decided to just have a go myself and see what i found.
First of all i just plugged the head unit in exactly as the original. the 10pin speaker connector doesnt fit, so i made up some short jumper wires to test things out.
This gave the first set of findings. A) the door amplifiers buzz like mad if they are powered but have an open input. For testing you can simply short the two leads in the plug and the buzzing will stop. B) driving the door amplifiers with the "speaker" output of the Pioneer worked fine, there was no buzzing or noise or anything weird, but it was VERY loud. On that particular unit "30" is usually a decent listening volume, and "20" is quiet. With it connected to the P38, 7 was loud, and 10 was rattling the door cards....
As expected, the speaker level output is much too high. Much to be expected, the door amps clearly expect a much lower level input.
A friend suggested i try using one leg of the speaker out from the head unit, and pull the other leg to ground. This is because most modern head units have the speaker outputs wired in a "push pull" fashion with a pair of transistors pulling the signal in opposite polarity. Using one output and ground would thus half the voltage. This worked, and did make it slightly quieter, but it was still too much, and also introduced a hiss (albeit a fairly subtle hiss) at lower volumes.
So i set that idea aside, and instead investigated using the "pre-outs" from the head unit. Clearly this is the "correct" thing to use, its intended as a line level output for driving another amplifier. For testing i cut the ends off some phono leads, crimped on some ferrules and stuffed them into the 10 pin connector on the range rover harness. Hooked it up, and well, it just worked. Nice crisp clear audio, no messing about with resistors and all that mess as posted elsewhere.
I just needed to make something a bit more permenant without making a mess of the wiring. I initially wanted to get a matching plug for the 10pin connector on the car, however they seem a bit rare and awkward to get hold of, and i was being impatient, so i grabbed a spare Deutsch DT20 series from the box at work and set about with the crimpers.
Replaced the car connector with a 12way DT, and made up a lead with a matching socket going to the 5 phono leads.
With that done, i had to repin the pioneer power flylead to get it to feed the "remote on" signal to the correct pin to turn the amps on. For some reason landrover didnt use the proper wire for that.
With that lot done and a final test, it all works as expected.
I now need to find a minute to install the microphone for the bluetooth part then i can get some photos of the finished installation.