The rule
German r does two different things, and the position tells you which. At the start of a word or syllable it is a real r, made at the back of the throat: rechts, Richtung, Straße. At the end it softens almost to a vowel — Uhr comes out closer to "Uh-a".
r by position
| start — rechts, Richtung | throaty r |
|---|---|
| end — Uhr, Tür, hier | soft, almost „uh" |
| -er ending — Vater, Zimmer | „a" — Vat-a, Zimm-a |
Trying to roll every r the English or Spanish way is the single most audible foreign accent in German. At the end of a word, do less, not more.