In Australia, a poorly maintained/rusty car is called a "bomb" or a "bomby car" and described as "bomby".
'Look at that bomby Falcon - it's the rustiest thing I've ever seen!'
'Wow, that red Corolla's an absolute bomb!'
From Wikipedia:
In Australian slang the terms rust bucket, 'bunky', old bomb, paddock basher or bomb are used to refer to old, rusty and/or rundown cars.
bomb
noun
(Australia/New Zealand, slang) an old or dilapidated motorcar
Source: Collins English Dictionary