signed up to post this. found a SOLUTION (came across this thread having had been googling the same problem). 'SMS, my car and me' works! and with android 5. couldn't get any other apps to work, but finally found this app. it cost me £3.25, which is the only downside, but it's not that much really, price of a pint or a sandwich! thought i had it all set up right to play messages over car BT audio, but could only get it to work whilst streaming music from my galaxy S4 via car BT. so no good for if i'm listening to radio or CD (which i am 95% of the time). then enabled 'send output to bluetooth headset' under preferences/audio settings - forwarding. now with that mono setting enabled, it works. it sees/sends the text as a typical mono phone call, thus muting the radio/CD and reading out the text. there's loads of settings, including an auto-reply message - that, if you want, will only reply to texts from people in your phone book.
the only thing i'm not sure about, which some of you guys have mentioned, is the ability to reply to texts, rather than an auto-reply. there is an addon/3rd party app by the same author called SMS+Car offline Voice Control, didn't come up with a search on google play but found it searching 'SMS car' on google play store. it's still pretty minimal in its use, though. you can say 'call back' and it will call the sms sender back, or send them one of three predefined messages. so just to confirm, no support for full texting back over voice. but for just reading messages, it's a god send!!
hope this reply helps someone, at least. i know it's an old thread. and no, i am in no way affiliated with the author of this app. just a well chuffed bloke who now gets his messages read! oh, and there's a demo version if you're unsure, although some of the reviews said it's VERY minimal in use. i skipped it and just bought the app, so can't comment on the demo configurablility.
can't post links as i'm a new member. good luck guys