Cable for a Braun T1 radio?  

You need to know the input...
You need to know the input voltage and either the wattage or amperage. It probably says this somewhere on the device. But you could also get the information by looking up the battery's specifications.
The you need a transformer. The rated voltage should be at or below, but not over the expected voltage. And the rated wattage should be at least 20% above the expected.
You will have to figure out an elegant way to connect the wires off the transformer to the device.
Best of luck!

Does it actually have a input...
Does it actually have a input jack for electrical somewhere?

I have one of these as well...
I have one of these as well and couldn't get it working with a bettery and gave up. That said, check out this link....it may be helpful
http://www.radiomuseum.org/r/braun_transistor_1.html

Yes it has, Leif.
I've taken a pic. I assume the top one is for linking it to the car lighter
(I've read this was its extended role).
On radiomuseum.org there's some technical info and a pic of the connected radio, but is says it can take up to 220 V (am I right? so no need for a transfo??)
http://www.radiomuseum.org/r/braun_transistor_1.html

You beat me to it, Jesgord 🙂
I remember you had one like mine.

If it can take 220v then you ...
If it can take 220v then you don't need a transformer. (It must have an internal transformer to step down to the battery voltage, because there is no way the battery is outputting 220v).

BJ
I have a PDF of the T1 service manual. It's in German and some of the fine lines in the schematic are hard to see, but maybe it'll be helpful to you:
http://www.dropbox.com/s/gm05j7m3neuvktx/braun_t1_sm.pdf

Great stuff Fastfwd!
Only, it's a different T1 🙂 25 years older...

Oh. Whoops!
Sorry, BJ. I only saw a corner of it in the photo you posted and didn't notice that it's obviously different from the one in my manual with LED displays. How embarrassing.
I'll search for similar documentation of the device you ACTUALLY have. If I don't post it here in the next day or two, I didn't find it.

Ok, here's a schematic.
Note in the lower-right corner that there is a mechanical switch of some sort to select between 110V, 125V, 190V, and 220V mains. Note also that when it's running on battery power or from what is alleged to be a car cigarette-lighter input, it requires 6V, not the 12V that is produced by most modern car cigarette lighters.
http://www.dropbox.com/s/yblqeh1knj5m64h/Braun%20Transistor%201.pdf

And...
Leif or BJ, is there a jack for an external antenna on your radio?

BJ, Could you take a picture...
BJ, Could you take a picture of the battery compartment of your radio with battery in and out? I'd like to see if mine is missing any parts connections and make sure I am putting the battery in the right way.

Certainly Jesgord,
was away yesterday (had to pick up an Olsen piece, see latest...) but took a snappy this morning.

Thanks! The battery...
Thanks! The battery negative/positives springs face downward when the battery is installed, correct?
Jesse