After a lot of experiments on the issue, I think I've found what I can call a "best solution". I came across it by chance, experimenting on different chemicals that would remove that awful sticky material without harming the colour of the plastic. Well, the best results came from the oven cleaning spray called "FORNET" in Greece (I think it's a JOHNSON'S product), but I guess that every other oven cleaning foam would do the job. The instructions say that you must use it in a hot oven, and that indeed stands for AR's "dirty plastics". I heated them with a hair dryer until I could barely touch them and then gave them a generous foam spray. I let the foam for about 15 minutes (a 2 cigarrettes wait for those who smoke), and then starting rubbing them thoroughly. Don't use any kind of cloth. I use common furniture foam 1mm thick (it is sold in 1meterx1meter sheets with a cost of 5 euros a sheet in my country. Of course you will need three or four spray coatings for every part, and accordingly a lot of foam to remove it completely.
Bear in mind that the worse condition the plastic is, the easier it is to remove the sticky material. If it has just began to "stick" leave it until it becomes worse.
In one case, after trying to remove that residue from the central heating vent (on top of 147's fascia) which had just begun to "stick", and after alot of foam coatings and rubbing, there was a small decolouration in one part. Well the two picures attached is the remedy. The first (ending in 28) is a DUPLICOLOR's "spritz spachtel" (in German), an acrylic coating which you can spray after treating the part with a smooth sandpaper (600 first and the 1000). The other is a DUPLICOLOR's "structural effect" spray (black or clear) which you can spray on top of the first (after having completely dried). It is a spray which gives a "grainy "look on the plastic like the original. If you spray it from a distance of 30-40cm the effect is smoother, but from 50-70cm the grains are rougher. You have to experiment. I insist on DUPLICOLOR sprays. After using them for many years I can tell you that they are the best and especially for in-car applications. I've painted things in various cars 10 or 13 years ago with no damage at all . You send me an e-mail for every advice especially in any 147, 156 interior.