As I said, ask Count von Stauffenberg. He was there, tried that, unsuccessfully, to his terminal detriment less than 24 hours later.
Not all German assassinations have been such fiascos. For instance, consider the murder of Deutsche Bank president Alfred Herrhausen in 1989, ostensibly by the Red Army Faction, but probably by Stasi (or maybe it was team effort).
The assassins positioned an EFP in a bicycle saddle bag pointed at the road their target would be taking to work. They rigged an infrared beam detector the appropriate partial car length ahead of where the target would be sitting at the instant his limo broke the beam. The saddle bag bomb needed to be armed before it would respond to the breaking of the beam.
The arming of the bomb was accomplished by remote control by an observer. Herrhausen left for work at 0830 in a three-car motorcade, consisting of a lead car, Herrhausen's limo, and a trailing car. When the observer saw the lead car pass by, he armed the bomb. The instant Herrhausen's armored Mercedes broke the infrared beam, the bomb detonated, driving two kilos of molten copper through the armor and into the passenger seat. The be-legged Herrhausen died of blood loss before help arrived. His driver, who was ahead of the explosively formed projectile's impact point, survived.
Note that the scheme didn't need any stinkin' Casio watches or Radio Shack Micronta clocks. It needed careful placement of the bomb the correct distance from where the beam would be broken. And it needed an observer to arm it when the next vehicle was the target's.
Hey! All you English teaches out there! If one of your kids has one of these in his backpack, you might want to call the principal:
 An EFP |