Have you replaced the coax cable? It may have been squashed which changes what is known as the characteritic  impedance which in turn causes the efficiency to be drastically reduced as I found with the HF vehicle radio when I worked at Derby many moons ago. The other techs just put up with it. The inner conductor in on spot was nearly through the dielectric/ insulator and when the coax was replaced everything worked fine

And if you do make sure it has the correct impedance for use on radios. It must be 50ohm and not 75ohm which is for TV reception.

Your sea rescue may have the gear to test your set.

Hope this is not too technical and is of some help. I worked for 34 years with PMG/ Telecom / Telstra and gained quite a bit of expertise in this area.

Mike

Thanks Grover,

Checked the cable which was bought and installed at the same time (GME Mount with cable attached)

I suppose one way of knowing for sure is if someone near the ocean has a Unit setup in their boat and can plug it in and get a Radio check from Sea Rescue

 I would say its probably got water in it.  If its got the die cast back and plastic front. The ally corrodes and water gets through the seal. You can pull them apart pretty easy and have a look inside.   Sounds similar to what mine was doing and when I opened it it had green death on the cct board. The only other thing is check power to it the connector can rot or bad connection at the fuse or switch. That will cause it to power off and low signal.

 Check the fuse holder--those little glass fuses in their spring-loaded holders are very prone to getting a little ( or lot) of corrosion, and the resulting voltage drop will play hell with them, could easily give the symptoms described.

t

you need to check all of the following,  all coax and antenna connections are clean and in good condition , then all of the power cables and fuses are clean connections and making good contact , the only way to ensure you have full 12 volts DC to the radio is with a volt meter connected whilst you are transmitting , the voltage needs to stay up at 12 volts .

you also need to check you are transmitting on high power. VHF 25 watts  and rather than doing a test call from your drive way go down to the coast area , clear of any limestone and try and contact your local sea rescue for a radio check , a lot of items interfere with Vhf signals so you need to take all of these out of the senario , other wise take your rig to a company who can test the output power of the radio and antenna  

An Update:

I traced the cable behind the dash to a corner where it turns 90 degrees and it was completely frayed and mostly severed

Hoping it hasn't damaged the unit due to the antenna not being hooked up directly

Anyway feeling pretty embarrassed over something simple and appreciate everyone's feedback & ideas

I'll let you know the outcome once I replace the lead

a few weeks ago at the Abrolhos I was having trouble with my VHF, couldnt get Geraldton via the repeater.
Discovered a few hours later that the problem was one of those little USB cig lighter adapters we were using to charge the phones.
Unplugged and boomed thru crystal clear.

 Hi. If the cable is long enough you can put a new end on them. If it hasnt had salt water in the cable and you can cut back to fresh shiny uncorroded cable an auto sparky will solder a new end on for around 10 dollars.  Cables are usually pretty long and you can cut them back.  

With the mount only costing $39 I replaced it this afternoon

In my Driveway in Maddington I reached VN6DI on 73 5 by 5, i'd say that's fixed and working

Thanks again for suggestions and help and to Compleat Angler Nedlands who contacted me and offered assistance to get this fixed, that's service!