where is this control unit? which could be the problem?Weeks ago the beep became lower and finaly there is no beep... and dont know what the matter.
Thank you again 
speaker failed on another elite i have here. I suppose the tell tale would be if the sensors tick audibly or to a light touch, but no tone is heard in the car then good chance the speaker has failed. But i diagnosed mine with an electricians meter which will show a varieing current at the speaker plug with reverse gear selected and ignition on. Engine does not need to be running.
Control unit, remove spare wheel for access, its on the outboard side of the vertical metal panel in the space between panel and rear wing, 2 screws are easy to get to.
Speaker wire has a black braided fabric cover, with a plug covered in foam wrap cable tied to the same metal panel, if faulty unplug it there then from the rear seat remove the c pillar cover on the spare wheel side of the car, you'll see the braided wire running up to the roof lining, unclip the roof lining enough to pull the speaker out, its just stuck to the roof in a lump of foam, exact location seems to varie, but just follow the wire.
If you need to remove it from the car to solder on another speaker then wire will pull through the rear seat bulk head, it will stick but a firm tug and the plug will pull through.
I dont think re fitting is possible in the same route, so i fed the cable back through a hole in the seat bulk head by the seat belt track, then feed the cable to the unit that way instead.
hth