Well, the other expression is much more colloquial, you will not find it in a dictionary or somewhere. Grundig was a well know producer of electronics, radios and TVs, founded in 1930 by Max Grundig and a big player during the Nazi time and the 1950s and 1960s "Wirtschaftswunder" years. But then it's quality and innovation increasingly deterioted, until they went into bankruptcy, in the 2000s. Some parts may still exists with other names in different companies. However, "schundig" means trashy, shoddy, so this phrase was just a rhyme to poke fun of an apparently nice product (of the high reputation epoch of Grundig) with low-level interior.
So what about this attempt (trying to copy the use of a brand name): Outside Mytek, inside car wreck?