I have installed Custom field in checkout plugin and as advised by documentation I added following code
<li class="fields">
<div class="field">
<label for="billing:ssn" class="required"><em>*</em><?php echo $this->__('SSN') ?></label>
<div class="input-box">
<input type="text" name="custom[ssn]" value="<?php echo $this->htmlEscape($this->getQuote()->getSsn()) ?>" title="<?php echo $this->__('SSN') ?>" class="input-text required-entry" id="billing:ssn" />
</div>
</div>
</li>
to persistent/checkout/onepage/billing.phtml
and to every billing.phtml
I found, just to make sure.
However the new field doesn't show up anywhere, ctrl+f with <input type="text" name="custom[ssn]"
on checkout page's source finds nothing.
I'm using the newest Magento version (1.9.0.1).
I'm open to other methods of adding custom fields, provided I won't need to touch Magento core files.