1977 Mercedes Benz 300d, exceptional condition

1977 Mercedes 300D 47k original miles. I have the original buyers order from 1977. I'm the 3 rd owner of this car. I have all documentation to show miles are true. What I've done. changed glow plugs to modern style. new glow plug relay. all fluids changed and flushed. re bolstered back seat. electronic heat and ac control. new battery. This car still has its original spare tire. 2 extra matching hub caps. there is 0 rust anywhere. interior is near perfect. Replaced door seals and door latches. What doesn't work. the cruise control I haven't been able to locate an amplifier. everything else works including power antenna and original AM/FM stereo. Also have 100% complete first aid kit