For example, we have - car /kɑː(r)/ - or /ɔː(r)/
I thought the brackets means you delete it - i.e. non rhotic - but now I see the phonetic spelling of words like "hard" which don't include the r at all /ˈhɑːd/
There is a basic rule for the pronunciation of /r/ in non-rhotic varieties of English, for example Southern Standard British English. We only pronounce /r/ if it precedes a vowel sound (sounds, not letters are the important factor). So in standard British English we see the following:
In the words above we see orthographic, written, 'R' followed by vowel sounds in carrot and road and so it is pronounced in these words. In park, however, 'R' is followed by the consonant /k/, and so is not pronounced. Notice that if an 'R' is followed by silence it isn't followed by a vowel and is therefore not pronounced. Also notice that the following vowel can be in a completely different word. So we see the following types of pronunciations:
kɑ: pɑ:k (car park)
kɑ:r əlɑ:m (car alarm)
In the first example, there is no following vowel, so we don't pronounce the 'R' in car. In the second, the 'R' in car is followed by the consonant /p/, so again we don't pronounce it. In the third example this 'R' is followed by the first vowel in the word alarm and so this 'R' is pronounced /kɑ:r əlɑ:m/.
Lastly notice that written vowels are not important. So in the word weren't, we find:
The last 'E' in weren't is, of course, silent. So the sound following the orthographic 'R' is the consonant, /n/. Because of this we do not pronounce the 'R' here.